Will my friends and I be able to play STALKER 2 offline on our LAN?

Or will we be forced to use GOG Galaxy or something to play on centralized servers?

There will be no multiplayer on release. It is supposed to be added in a future patch, though. However it will probably take a while before this will happen. How exactly the multiplayer is going to work is currently unknown.

Most videogames stopped having LAN years ago so i am guessing that Galaxy will be needed. Either way you should have Galaxy installed for new games since most need constant updates and patches soon after release.