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My understanding is that LGM has deployed an big update/complete version of SPG3 on the GOG server, and that verification checks out, but when a download occurs a wrong version is sent back from servers again, and version mismatch repeats itself. In my case it seems to be around 42 megs.

LGM has reached out to the tech team at GOG, hopefully we'll see a fix soon, especially with SPG3 on sale, likely getting more downloads than usual.
Post edited September 27, 2020 by Joseph71
That is unfortunately true at this point. Version deployment is having issues on GOG. We're working with guys from GOG to find a solution. We had some improvements today, but not ultimate solution yet. We will inform you all as soon as difficulties are resolved and version can be normally updated! Big apologies for this!
According to our recent tests, GOG version should now be operational to all players. If any of you experience any further issues with starting up the game, please notify us!