Aeternull: Why were these games removed from GOG?
Various reasons. In case of Re-Volt, the publisher had taken a community patch without asking for permission to make the game run on modern systems. Telltale went belly up and so did their presence on GOG.
Everything was delisted because they didn't want to update the game. Another title with update problems was
Wolcen: Lords of Mayhem. Going by the comments,
Necropolis was "a piece of crap". And sometimes a publisher simply loses or sells the rights, like
Lula: The Sexy Empire and Industry Giant 2.
In case of Duke Nukem, "Pitchford is poison" is what I've heard.
tinyE: Colin McRae
and I'm still super fucking pissed about it!
Lucian_Galca: Same here considering how few racing sims there are on gog. Only reason I didn't grab it at the time was because I didn't expect to be able to afford a high end gaming pc build, but I ended up doing just that.
Racing sims often feature licensed cars, which seems to be a problem. Nearly all games (or maybe just all games?) with cars on GOG have fantasy brand cars. Quite some racing sims also have licensed music which may be an issue. I think some music was removed from Mafia 1 and Quake 1 and 2 (not racing games I know) here because of that.