Yesterday I completed Close to the Sun and the speedrun achievement (under 3hrs) didn't unlock so I contacted gog support and linked to the Steam post the dev made about removing the 2 achievements there .. seems they removed them too from gog version

The Crimson Diamond does have working Galaxy achievements, however a Galaxy.dll is missing from the current build. Since it's an AGS game, I copied the missing Galaxy.dll from The Shivah and uploaded the file here:
https://www.mediafire.com/file/2o24rvzwnomles6/Galaxy.dll/file
Just copy the file into your default Crimson Diamond installation directory and you are good to go.

Yes, 5 Co-op achievements can't be obtained.
Should be included in Excel file.