Assalaamu alaykum. I have heard that when Pharaoh was about to die, the angel Gabriel put dirt in his (Pharaoh's) mouth so that he would not be able to pronounce the words that would allow him to die as a believer. Is this true? I ask because I am confused, if the angel Gabriel can do such, then does this not mean that Gabriel has free will? I thought that angels did not have free will.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

Ibn ‘Abbaas, may Allah be pleased with him, narrated that the Prophet, sallallahu ‘alayhi wa sallam, said, "When Allah drowned Pharaoh he said, 'I believe that there is no god except the One that the children of Israel believe in.' So Jibreel (Gabriel) said, 'O Muhammad! If you could only have seen me while I was taking the mud from the sea, and filling his mouth out of fear that the mercy would reach him.'" [At-Tirmithi graded it hasan (good) - Al-Albaani graded it Saheeh-Lighayrih (sound due to external factors, such as another hadeeth with stronger chains of narration corroborating it)]

The version narrated by Ahmad and Ibn Hibbaan reads, "Jibreel was shoving clay in the mouth of Pharaoh out of fear that he would say 'La Ilaaha Illa Allah' (None is worthy of worship save Allah)." Ahmad Shaakir and Husayn Asad declared its chain of narration to be authentic.

Thus you come to know that the story is authentic (true).

As for the action of Angel Jibreel, it was done at the command of Allah, The Exalted, as underlined in Tafseer Al-Khaazin, which reads, "Allah, The Exalted, prevented Pharaoh from believing in Him before his death as a punishment for his past disbelief and his efforts in driving people away from Him. As for the action of Jibreel, shoving clay in Pharaoh's mouth, it was done at the command of Allah, not out of his own accord." [Tafseer Al-Khaazin, 2/461]

You should know that angels are a creation of Allah, The Exalted, whom He made submissively obedient to Him, and they comply with His commands and carry out the duties that He assigns to them. Allah, The Exalted, says (what means): {...they do not disobey Allah in what He commands them but do what they are commanded.} [Quran 66:6], {They cannot precede Him in word, and they act by His command.} [Quran 21:27] and {They fear their Lord above them, and they do what they are commanded.} [Quran 16:50]

All these verses indicate that the Angels do not do any action except with the command of Allah and His permission.

Allah knows best.