If I would have to guess I'd say GoG copies are legal. If not they would have to use a key generator for the license keys and soon buyers would run into "key already in use" messages if they go online. (yes I know GoG sells the game mostly as single player game, but I am sure some buyers would try to find out if they can still play online.)
Selling a pirated game would be a very stupid move by GoG because for a few dollars profit they would risk huge problems and fines. So I don't think they would risk it.
As in the interest of GOG, they'll never sell a pirate copy of a game. When they do it ever, they ruin themselves. I also would prefer to contact directly 2k Czech on
because they're holding the rights on Hidden & Dangerous 2 as successor of Illusion Softworks (as far as I know).