KeeperFX does not work for me. I have installed the GOG version, extracted KeeperFX in a separate folder, and tried to use game.gog as an iso to put all the necessary data filed into the KeeperFX folder.
However, the game will not work and will only display an error message suggesting to read the log, which refers to missing files in the data folder. Using the launcher to get the files from the mounted iso also mentions the absence of these files.
So I have tried to open the iso directly with 7z, IZarc and browse through the mounted drive via daemon tools. 7z plain refuses to open the file even after being renamed as a .iso, IZarc and Daemon Tools both can't find the missing files.
I tried reinstalling the game using the GOG installer but it seems the game.gog file is either corrupted or not the right one.
7zip will not open the file.
My game.gog file has a CRC32 checksum of 105F8633 and is 120 MB (126,669,312 bytes). Can you confirm that yours is the same?
Did you rename it to game.iso? I highly recommend trying different software to mount the ISO to see if you get the same results.
I doubt that GOG have tampered with the file in any way because DOSBox itself mounts the game.gog internally, along with the OGG files as audio tracks.