After Attack on Titan episode 77, the fandom (mostly consisted of fans who can’t stand Gabi Braun) believed that Gabi was close to redemption. Seeing the casualties and the consequences of war, Gabi briefly decided that the war only brings horror and terror, renouncing her place in the battle. We could also witness Falco admitting to Gabi that the only reason he enrolled in the forces was to keep her from getting killed.
Falco even admitted his love for Gabi and his dream that they would one day marry and have children. Gabi was mildly disgusted by the idea of marriage, but that didn’t affect her realization that the war is futile and that it hurts the innocent the most. What we could see in episode 78, titled Two Brothers is Falco’s brother, Colt, beginning Zeke who survived the shot that could have been fatal, designed as anti-Titan weaponry, not to scream.
If Zeke screams, every person who drank the spinal fluid will turn into a Titan, which is a part of Zeke’s offense and defense strategy. Judging by the number of Titans he will get, this is more likely an offense strategy that brings advantage to Zeke and his cause. Of course, Falco drank some of the fluid, which means that if Zeke screamed, he will also turn into a Titan to Gabi’s and Colt’s horror. Zeke explained to Colt that he appreciates his bond with his brother, but screams regardless, waking up dozens of Titans as far as the eye can see. Falco also turned into a Titan.
Driven by the pain of seeing Falco as almost a mindless flesh-eating Titan, Gabi snaps and takes a gun, which is the moment when she snaps and reclaims her place in the ongoing war. Seeing Eren running away from his Titan and far from Reiner’s Titan who is being attacked by Falco’s Titan, Gabi shoots at Eren. This is the moment where she is back to her revengeful motif as she loses her friend to the war.