It is widely accepted in Muslim thought that 'Jibril' (henceforth 'Gabriel') is an angel or an important part of the creation termed 'malaika'. From a Quran's perspective there is no unequivocal statement which corroborates this well held belief and as the article will attempt to show, there is considerable evidence to contend with this well established view.
Gabriel is mentioned 3 times in the Quran by name.
"Say: Whoever is the enemy to Gabriel (Jibril) - for indeed he revealed it to your heart by God's permission, confirming that which is before it (co-existent) and a guidance and glad tidings for the believers"
"Whoever is an enemy to God and His angels and His messengers and Gabriel (Jibril) and Michael (Meekael), then indeed God is an enemy to the disbelievers."
"If you both turn to God, then indeed your hearts are already inclined (to this); and if you back up each other against him, then indeed God, He it is Who is his Guardian / Protector, and Gabriel (Jibril) and righteous believers and the angels after that are his assistants."
One notes that in two verses above (2:98, 66:4) the category of the angels have been seemingly treated as distinct and separate from Gabriel. The conjunction 'wa' (and) seems to separate the different entities as follows:
(1) God (Allah)
(2) Angels (malaika)
(3) Messengers (rasuli)
(4) Gabriel (Jibril)
(5) Michael (Meekael)
(1) God (Allah)
(2) Gabriel (Jibril)
(3) Righteous Believers (Salihul'mu'minina)
(4) Angels (Malaika)
However, it is to be appreciated that it is not unusual for the Quran to mention a member of a category separately to provide additional emphasis despite the conjunction 'wa' (and) apparently providing separation.
For example, in the following verse, one notes:
"In both of them (are) fruits (Arabic: fakihatun) and (wa) date-palms and (wa) pomegranates"
Here the conjunction 'wa' (and) when used with date-palms and pomegranates only clarifies the 'fruits', seemingly providing additional emphasis and is not read / understood as separate from the category of fruits (fakihatun).
Another example is noted in the following verse:
prayers (salawati) and (wa) to the middle prayer (salat-ul wusta) and stand up truly obedient to God.
Once again, a particular salat has been singled out for specific emphasis from the general category of prayers.
At this point, the above verses would not provide sufficient warrant to provide a satisfactory conclusion as to whether Gabriel was an angel or not.
IS GABRIEL SYNONYMOUS WITH THE HOLY SPIRIT?
It is once one compares verse 2:91 with verse 16:102 of the Holy Quran, that a striking overlap is noted whereby both Gabriel and the Holy Spirit are mentioned as being responsible for revealing the Quran by God's permission.
"Say: Whoever is the enemy to Gabriel (Jibril) - for indeed he revealed it / brought it (nazzala) to your heart by God's permission, confirming that which is before it (co-existent) and a guidance and glad tidings for the believers"
"Say, the Holy Spirit (Ruh al-Qudoos) has revealed it / brought it (nazzala) from your Lord in Truth, in order to strengthen those who believe, and as guidance and glad tidings to Muslims"
From the verses above, one would have to strongly posit the suggestion that Gabriel is synonymous with the Holy Spirit and is not from the category of angels.
In the following verses, if one allows for the different descriptions of the 'Ruh', one notes the strong suggestion that the Spirit remains a separate entity from the general category of angels. This is also discussed in a dedicated article. Please see  below.
"The Day that the Spirit (al-Ruh - singular) and the Angels (al-Malaikatu - plural) will stand forth in rows, none shall speak except any who is permitted by the Most Gracious (God), and He will say what is correct"
"He sends down the Angels with the Spirit of His command (Ruhe-min-amr) to whom He will of His bondmen / slaves, (saying): Warn mankind that there is no God save Me, so keep your duty to Me."
"The Angels (Malaika) and the Spirit (al-Ruh) ascend to him in a Day the measure whereof is (as) fifty thousand years: "
"Descend the Angels (Malaika) and the Spirit (al- Ruh) therein by the permission of their Lord, on every errand"
In another verse, the Holy Spirit also seems to be referred to as the Faithful / Trustworthy Spirit and once again, responsible for revealing the Quran to the Prophet. This once again strengthens the suggestion that Gabriel is indeed the Holy / Faithful Spirit.
"And indeed, it surely is a revelation from the Lord of the worlds. The Faithful / Trustworthy Spirit (Ruh al-ameen) has revealed it / brought it (nazala) upon your heart that you may be of the warners. In plain / clear Arabic language"
OVERLAP WITH BIBLICAL NARRATIVES
In the New Testament, one finds an overlap of a narrative with the Quran which concerns Mary and where Gabriel is mentioned by name.
26 And in the sixth month the angel (Greek: Aggelos) Gabriel was sent from God unto a city of Galilee, named Nazareth
27 To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin's name was Mary
28 And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women 
29 And when she saw him, she was troubled at his saying, and cast in her mind what manner of salutation this should be.
30 And the angel said unto her, Fear not, Mary: for thou hast found favour with God.
31 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS.
32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David
33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
34 Then said Mary unto the angel, How shall this be, seeing I know not a man?
In the corresponding Quranic narrative, one notes only the mention of the 'Spirit' (Ruh) and not any reference to an 'angel' (malak).
"Then she took from them a screen; then We sent to her Our spirit (Ruhana), then he assumed for her the likeness of a well-proportioned / well-made / perfect man. She said: Indeed I seek refuge from you with the Most Gracious, if you are one guarding (against evil) / God fearing. "He said: "Indeed, I am only a messenger from thy Lord, (to announce) to thee the gift of a pure / innocent son". She said: How can I have a boy when no mortal has touched me, nor have I been unchaste? He said: Even so; your Lord says: It is easy to Me: and that We may make him a sign for humankind and a Mercy from Us, and it is a matter which has been decreed."The mention of the Spirit (19:17) seems deliberate which remains in stark contrast to verse 3:45 below where 'angels' have been clearly cited covering a similar incident yet pertaining to a different period of time. 
"When the angels said: O Mary, indeed God gives you good news with a Word from Him..."
Therefore, it seems that is was not an angel that visited Mary in the incident covering the Quranic narratives in verses 19:17ff, but a 'Spirit'. If the Biblical perspective is accepted whereby Gabriel visited Mary to give glad tiding of a son, then the Spirit mentioned in the Quranic narrative would imply it to be Gabriel. Such a synonymy of Gabriel and the Holy / Faithful Spirit has already been noted in the Quranic verses cited above (2:97, 16:102 and 26:192-195).
However, in verses 19:17ff, it appears that the Quran deliberately avoids using the term 'angel' in the comparable narratives concerning Mary when it could have done so as in verse 3:45. This is 'possibly' with a view to depart from the Christian understanding that Gabriel was in fact an angel.
UNDERSTANDING VERSE 77:5
It is often asserted by some critics (especially from critical Christian corners) that verse 77:5 alludes to multiple Divine agents responsible for revealing the Quran to the Prophet. Such an argument is usually posited to counter any suggestion that there was only one Divine agent responsible for the task of revealing the Quran to the Prophet based on the interpretation solicited from verses 2:97, 16:102 and 26:193.
Quite apart from absolutely no mention of angels in the Arabic text of verse 77:5, the verse is best rendered implying those that simply spread the message of the Quran.
In verses 2:97, 16:102 and 26:193, the verb used to denote the revelation of the Quran to the Prophet is 'nazzala' / 'nazala'. The is clearly not the case in verse 77:5 where the verb 'alqa' is used which means to cast, throw, hurl, to convey or to offer.
Illustration - Joseph Islam
Therefore, in the context of the verses, there is no warrant to conclude that this passage refers to multiple angels or pertains to the revelation of the Quran to the Prophet. Therefore, the Quran's position remains clear that only one Divine agent was responsible for revealing the Quran to the Prophet by Godís leave.
There is no Quranic warrant to assert that Gabriel was in fact an angel or from the category of angels. The Quran rather, seems to suggest that He was of a different entity and possibly the Spirit referred to in the Quran by different names such as the Ruh al-Qudoos (16:102) or the Ruh al-ameen (26:193). It is not within the scope of the article to provide a detailed analysis of the Quranic term 'Ruh'. Please see the related article (1) below.
It is important to remember that God's creation is both diverse and immense and remains scattered throughout His Universe (42:29) and possibly beyond. He further adds to His creation as He wills (35:1). It would be a restriction on His Majesty's infinite power to suggest that His creative realm only consists of humans, animals, the jinn, angels and the Holy Spirit.
In the end, the Quran only furnishes information which is relevant for mankindís guidance.
(1) What is the Quranic Ruh (Spirit)
(2) How Many Angels Were Talking to Mary?
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