S! Stratos, Swordfish is correct.

The only difference between GOG's version and the Sierra version is a modification to the exe's so it does not ask for the CD. In SP, that is not a problem, in MP, as Swordfish stated, that causes problems with secure servers in MP.

I run Revenge of the Jastas with my GOG version with no problems.

GOG also has a forum for Red Baron which i frequent alot, so if there are any problems getting the game to run with Win7/Vista 64 bit, I may be able to help.