Content Licensing?

Hello. I am curious about what the content is licensed. I know that the engine source is GPL and the ripped assets, being ripped, can't be used in commercial projects. What about the rest of the content?

Original assets (excluding characters):
Characters (Sonic, Tails, Knuckles, Robotnik/Eggman, Metal Sonic):
gamecode:

The reason I ask is because I am curious if a hypothetical someone was to take the game and do with it what the freedoom and openarean projects did with doom and quake 3 respectively.

Like any fangame, SRB2 is a trademark violation. If SEGA wanted to shut SRB2 down, they could. Basically the only "open" part of the game is the source code, which is open source by the license of the GPL. All of the resource files are not open, meaning that they are owned either by the original creators or us, depending on who made the assets.

If for some reason someone wanted to make an "open" version of SRB2, they'd need to completely remake all of the art assets and make their own original mascot characters.