Direct proof of fielding
Bellingcat reported in February 2015 that the Syrian Arab Army had acquired two types of single-shot 40 mm under-barrel grenade launchers for AK-74M-era rifle use: the GP-25 and the GP-30M. The same report placed the AK-74M and related Russian small-arms deliveries in Syrian Arab Army and Republican Guard use during the civil war.
The available public source used here supports Syrian government acquisition and fielding of GP-25 launchers in the conflict. It does not identify a dated GP-25 firing incident, individual unit serial number, or specific battlefield location for a confirmed GP-25 launch.
Sources: From Russia with Love, Syria's AK-74Ms
Timeline
Before the war, Syrian modernization efforts around the AK-74M created the host-rifle context in which Russian under-barrel grenade launchers later appeared. Bellingcat described the Syrian Arab Army as testing AK-74M rifles in 2008 and then receiving larger AK-74M batches in the years around the civil war.
By 19 February 2015, Bellingcat had reported GP-25 and GP-30M launchers among the under-barrel grenade-launcher types acquired by the Syrian Arab Army. That makes 2015 the first dated public milestone in this record for GP-25 fielding in the Syrian Civil War.
Sources: From Russia with Love, Syria's AK-74Ms
Role in Syrian government service
The GP-25 is a compact launcher attached beneath Kalashnikov-pattern rifles, giving an infantry rifleman a single-shot 40 mm grenade capability without replacing the host rifle. Small Arms Survey identifies the GP-25 and GP-30 as under-barrel launchers designed for Kalashnikov-pattern assault rifles, while Modern Firearms describes the GP-25 as a Soviet 1978 design for AKM and AK-74 rifles.
In the Syrian record, that places the GP-25 in a government infantry fire-support role rather than as a standalone artillery or vehicle system. Bellingcat specifically connected the GP-25/GP-30M acquisition to Syrian Arab Army rifle use; the sources do not establish documented opposition, Islamic State, SDF, Turkish, Israeli, or coalition use of the GP-25 in this conflict.
Sources: From Russia with Love, Syria's AK-74Ms, SAS GP-25 weapons ID sheet, GP-25 and GP-30