Two questions remain: Was that huge package created intentionally and was it GOG or the developer who (mistakenly) inflated the installer?
Well I just asked DarkGod and got this reply:
I can confirm I sent them 2 450ish MB files, one for linux32 and one for linux64; no diea how they end up with 4GB :/
I think GOG packages their Linux installers manually, which is why they usually take longer to update and sometimes contain little blunders.
EDIT: I sent a ticket to support.