The Battle of Tazek'sur was a military engagement on Chulak, which was waged before the Tau'ri began exploring the galaxy and before the larger Jaffa Rebellion. The battle itself was a failed rebellion by Jaffa against the Goa'uld where many died, including Gerak's father.
This result of this battle may have been a loss for the Jaffa who incited it but it had another outcome where it helped to spread doubt among other Jaffa, including Gerak, to whether the Goa'uld were really gods. Teal'c used the remembrance of this battle to help Gerak recollect the Jaffa's past along with his own and turn him away from the Ori. After Teal'c and Gerak talked at the memorial site for this battle they went to Earth, where Gerak sacrificed himself to cure the prior plague. (SG1: "The Fourth Horseman, Part 2")