I have made the definitive choice to never support GMK 820, why? Because it does not even properly version itself. GMK 820's are GMK's exported from GayMaka: Stupido that version themselves as 810 but remove settings such as F4 for fullscreen and other options, and without the proper format version, it is impossible for me to tell whether or not to skip these bytes when reading the GMK.
To be honest, I am surprised at all anyone was even able to load these GMK's into GM81 exported from Studio. Studio is exporting crazy corrupt GMK's this way, and no comprehensive GMK reader would be able to read Studio's files. I would not export your games from Studio that way, they did just drop this feature too, and from what I can tell it was very buggy.
But anyway, in this screenshot you can see me looking at a GMK in a binary file viewer to check the version number, the version number is the second set of 4 bytes inside the file, but it also recurs throughout GMK.
See, look at the last part of the file is the resource tree...
The GMK format was horrible, that is why YoYoGames switched to an XML based format, GMX. So I will just finish my GMX reader and everybody can just be happy, GMK 820 isn't needed anyway when I finish GMX it will be able to export to any version of GMK specified.