A thing to point out....

[Revival9001]

Looking through old posts, I see things about people putting screenshots of srb2 on websites, and people replying with suggestions to sue. This is NOT possible. If a website puts up a download for srb2 that's ok because of the license. Now like it or not, srb2 is based of a gpl'd game so it must be under the same license. That license does not restrict redistribution.
Also, for people selling srb2: Probably a bit bad if it is unmodified. But if it's their own version of it, with code modifications, it can be sold, however it must include source and people who download must be allowed to make it availible for download, etc.

For those who wish to read the license, click here.

 

[Segmint]

Most of us already knew that. Oh, and they aren't allowed to sell it period. Sonic (and related characters, etc.) is under copyright of Sega. If you sell anything that has him on it, Sega must approve it. If not, it's totally against the law. Since the Doom Legacy engine is open source (I think...), it would be okay to sell it, just without Sonic and etcetera.

I guess.

 

[SSNTails]

Only the code is GPL, not the sound/graphics/etc.

You can redistribute the EXE, but not the other files with it. Sonic Team Jr. retains copyright on anything not falling under Sega Enterprises, Ltd.