Those little Chao critters will be showcasing NGC-GBA connectivity.
In the first confirmed, detailed case of a third-party developer exploiting the dual-system interconnectivity between GameCube and Game Boy Advance, Sega has announced that Sonic Adventure 2 and Sonic Advance will interact via the NGC-GBA link cable.
According to Euro-Asia Game Boy, the Chao Racing mini-game found in the Dreamcast version of Sonic Adventure 2 will also be present in that game's port to GameCube. However, the Dreamcast's VMU was used to train the little Chao creatures in "Chao Adventure 2"...on the GameCube, that feature will be accomplished via the Game Boy Advance. The original Sonic Advance title for GBA will be compatible with the GC version of Sonic Adventure 2, and hooking them up together will open up the mini-game, complete with the unique Chao of your choice from the garden(s).
Right now we can only assume that the mini-game will also be playable in some sense within Sonic Advance. This would allow owners of both games to train and take care of their Chaos on the road, then import them back to GameCube for use in the Chao Racing mini-game on the television.