Gabe's whole thing is attacking from long range so he'd mostly likely use his sniper rifle and get Rorschach in the knees/possibly a few other places because he's vindictive like that, and then get the others to help him close the distance. He really wants his pistol back. Which will most likely get messy as fuck, but I'm good with playing that out if you're interested. Or just having Gabe snipe him from a distance and then dealing with the fallout after, whichever you think would be most interesting.