Gabriel

(a Jibrāʼīlian soul), which bestows perception upon every atom of the atoms of existence;

just as instruction proceeds from it to universals and particulars, so too may outward instruction proceed from it, as happened to certain of the sanctified, such as the prophets.

And the most excellent of creation absolutely is our Prophet, God bless him and grant him peace, and his excellence is established even over our master Jibrīl and the rest of the angels brought near, and over others a fortiori — contrary to the Muʻtazilah, who claim the superiority of our master Jibrīl over the Prophet, God bless him and grant him peace.

But plainly the birth is announced by Gabriel.

Mary believed Gabriel.

or also the angel's announcing the virgin's conception and delivery?

It has been transmitted in the sound reports that Jibrīl, peace be upon him, came to the Prophet, may God bless him and grant him peace, while he was in houses whose doors were shut, and that he saw him in a form.

And He, exalted be He, said, "The trustworthy spirit brought it down" — and descent is among the attributes of bodies.

The spirits sent down from God, and Jibrīl, peace be upon him, are of this description.

that standing by the cross and seeing the things done and hearing the voices, after the testimony of Gabriel, after the ineffable knowledge of the divine conception, after the great display of the miracles, there will come, he says, a certain tossing about your soul too.

And while I was still speaking in prayer, behold the man Gabriel, whom I saw in a vision at the beginning, flying, and he touched me, about the hour of the evening sacrifice, and made me understand, and spoke with me, and said:

I recognize too the angel Gabriel sent to a virgin.