Dude Gaban was so in shock seeing Collun dying that it certainly affected his CoO and morale, he didn’t thought about anything else other than surrendering, specially cuz Gunko said "A Pirate King crewmate? Resistance means a hostage's death"
You could argue he could've faked his surrender and then fight back, but its tough because of the desperation, he was even having flashbacks of Collun's birth
He dropped his axe and was in tears, and even if he faked his surrender and counter attacked, he might've considerated that being so close to Collun and other children would still be too risky overall. After all, Sommers still used a few more stuff before going down and before Gaban says he feels rusty.
Really desperate situation, and shit happened way too fast. Katakuri's CoO got affected by getting too pissed by Luffy seeing his true face, just imagine how it affected Gaban here 💀💀