Ok for real I never knew what the cast was like for Planeteers because I was just a kid and now I'm stuck in the wikipedia page.

Kwame? LeVar Burton.

Gaia? Whoopi Goldberg (s1-3)

Rigger (Greedly's Henchman) was John Ratzenberger (Cliff from Cheers, been in nearly ever Pixar film).

Verminous Skumm was Jeff Fucking Goldblum in season 1.

Duke Nukem was Dean Stockwell. Dr. Blight was Meg Ryan. Tim Curry was her computer-thing.