it appears that its been folded into athletics (dexterity), as that is the skill you roll for to use non-recoilless weapons in zero G (ref pg63 example tasks, and pg77, environmental dangers).
when they did 2nd ed they changed a few skills around to simplify and streamline the skills needed (im guessing to make it easier to have all the skills needed to be effective in combat*). the Gun Combat skill got a major overhaul, and a few other skills got a few tweaks as well. I'm guessing the removal of Zero G Combat and Battle Dress as separate skills is part of that move. It might also be a way to reduce the penalties on a "non combat" character forced to fight.
quite of a lot of these changes seem to be "rationalisations", in that in 1E a PC could be a Olympic level gymnast, but be just as off balance as a PC with no athletic skill, or even worse off if that other PC has Zero G combat. since most of what the Zero G Combat skill covers is just gymnastics to move and fight in Zero G, it kinda makes sense to fold the two together.
*in 1e, a marine character with many years of experience using and fighting in BD would need Gun combat 1, Zero G Combat 1, Battle Dress 1, and probably Vacc suit 1, just to cover the basics, and wouldn't be able to use those fancy zero-g acrobatics in 1g. in 2e, that goes down to three skills, (athletics (dex)1, Gun1, vac suit2), and can use those acrobatic in normal gravity.