Upon the death of King Baldwin V of Jerusalem, a succession dispute arose between Guy of Lusignan, husband of Baldwin's heiress Queen Sibylla, and Count Raymond III of Tripoli, who had served as regent for Baldwin V and his predecessor. Gérard, harboring animosity towards Raymond, supported Guy, and renewed the war between the Kingdom of Jerusalem and Saladin of the Ayyubid Sultanate against Raymond's wishes.
When Saladin assaulted Tiberias in July 1187 in the hopes of baiting the Crusaders into a battle on an open field, Gérard urged Guy to march out from Acre to rescue Tiberias, spurning Raymond's suggestion it was a trap. Their forces were subsequently lured into the Battle of Hattin, which was decisively won by the Saracens and sealed the fall of the Kingdom of Jerusalem. Captured after the battle alongside Guy, Gérard was able to secure his release, only to be taken prisoner again at the Siege of Acre in 1189. This time, Saladin showed no mercy and had the Grand Master Templar beheaded.
Gérard's demise would result in a delayed election for his successor while the Templars debated the merits of selecting a Grand Master who served on the front lines. In the interim, Basilisk would pose as the order's de facto leader until Robert de Sablé's ascendance in 1191.