Hello. Maybe a stupid question. If i buy a game in GOG only (in my situation its Stronghold Crusader) can i play this game against people who use steam? I play on mac and there is no crusader on steam for mac, but i want to play with my friend who has steam version without using tunngle/hamachi. Is that possible?

AFAIK it doesn't use Steamworks so it doesn't matter if it's a GOG or Steam copy.
But I don't know how it does multiplayer, you might still need VPN software.

It depends entirely on the game: what forms of "remote" multiplayer it included, how the developers implemented them, how old the game is (which can affect whether any services the game's multiplayer required are still available), and some other factors.
Can't say specifically about this game, though, sorry.

i think stronghold crusader use direct ip and lan, no steamworks, so you can play with him