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.

I want to add to all seeing in this thread that as of 2024 this IS STILL UNFIXED
as a member of the X-COM community who knows and works with Filmboy84 and other members of openxcom and openapoc I can vouch that this is still the case and if you want to play without openapoc you will be facing this issue at some point

so this issue is unfixed at this time.
it can only be remedied by YOU

I will edit this message when or if this changes

OpenAPOC is right now not ideal for a playthough as its still in development and not stable.
(so dont say anything about it Darvond, you want the info ask Filmboy yourself)