I tried to play this game with a friend who also got it from GoG.com but... well, simply said, we're unable to host a game for each other for all we can. Does anyone knows which router ports needs to be forwarded to play this game?

From a quick google search:
UDP ports 2302-2400, 6073
Let us know if it works.

Thanks. I'll try it out. I tried google, but all sites that could have helped me were in foreign languages(speaks only english and french) .

Hrm. I'm able to get people joining by Gamespy, but my friend still seem unable to connect directly via IP. We're trying to hash out the source of the problem on his end, as he only uses a software firewall which he can(and did) disable for play.

Problem solved. The problem were indeed the ports(fixed thanks to the reply here) but also my friend's software firewall still preventing him to join after I forwarded my ports.

Glad I could help. Who knows, maybe we'll play together online some day. ;-)