Ausir: By the way, Fallout Tactics bonus preorder CD had an exe file that added a bonus mission (Springfield), but the mission was actually in the main game's files itself, and the file just modified the game's configuration file so that the mission is activated, which can also be easily done manually. I wonder if the GOG version has the ini file set to activate the bonus mission by default? If not, it would be a reasonable change to make.
You can do it by editing the "bos.cfg" file in the "core" directory. There is this line:
{misc.bonusMission} = {false}
Change the false to true, before the Mardin mission appears, i.e. you can finish the Quincy mission, but you have to change it before you go to Bunker Beta (or simply start a new game after the change).
I believe this may be coming later this week, as the file is already available on NMA, but is not currently accessible in fallout tactics.