How is it consistent that Gabriel (peace be upon him) did not remain with the Prophet (peace and blessings be upon him) until he heard what he heard, while devils eavesdrop from the heavens, and Allah Almighty says: "And We have guarded it from every outcast devil (17) Except one who steals a hearing and is pursued by a clear burning flame"?

There is no contradiction between the devils being able to eavesdrop and Gabriel not passing Sidrat al-Muntaha. The devils eavesdrop on what the angels speak in the lowest heaven. The narrations of the Isra’ and Mi’raj (Night Journey and Ascension) do not mention that Gabriel did not pass Sidrat al-Muntaha, but there are reports indicating that Gabriel parted ways with the Prophet, peace and blessings be upon him, at Sidrat al-Muntaha.