Since thread is up! by user above:

GOG is not related to Steam version. And this time Steam versions of X-com series is not related to GOG. For some titles - GOG did miss some exe (win executables) while steam is have them.

GOG Galaxy version really do not have mentioned here executables. Cant say right now for offline GOG build. But there shold be complete disc image - you can install\extract files from there and combine with *normally* installed offline GOG build.