It's drm, without a connection you cant save any items,weapons,etc. gog agrees since they took the game down from their store, otherwise would have kept it.
Question: does GOG have any game in their library that's a live services based game?
I mean, any game (single or multiplayer) that can't be played offline simply because of the server side part.
If the answer is "zero", then that's why.
In Hitman, any save, item, unlock, etc is stored on the server side, and it influences how you play every game in the franchise, much like what Mass Effect did in its time, but with no option to edit your saves (hence, impossible to cheat).
DRM = bullshit added AFTER the game is done to prevent copying and effectively owning any game.
Live service = bullshit implemented during game design so developer constantly delivers content and prevents cheating and stuff.
In DRM case, the game is complete and locked behind encryption methods (online or offline).
In live services case, half the game is on server side (much like an mmo), so playing it offline is simply impossible.
Why would anyone develop a single-player game as a live service is beyond me, but let's not call DRM something that was designed from the white board to be played online.