It sounds like you've got the mod's data files in the wrong place. Go into your game's root directory (where gcii.exe is) and make sure that's where you put the .sdf files such as "gc2reversal5.sdf" - there's one for each of the three modes. If they're sitting one level down in a different directory then you just need to move them over and it should work. Put the batch (.bat) files there as well, and then it should be as simple as starting the game by hitting enter one on of those.
Also make sure you downloaded the mod (from 2021 like you said) and not the source, if you can't find any .sdf then you might have the wrong package. This here is the download page for the source files and not the actual mod so hopefully that isn't the source of your problems!
Flamecurse
Richard joined
Mod developer of old games that (almost) nobody plays anymore.