We get a lot of e-mail asking if GTA3 is coming to the Cube as rumored. In short, no.
IGN Cube has contacted Rockstar to inquire about the rumors that a port of GTA3 is in the works for GameCube.
Rockstar's response was that other than the X-Box and PC versions (in addition to the already released PS2 version), there is not a GameCube version planned at this time.