NGC reported in issue 65 on page 14 that GTA3 would be appearing on GameCube this coming Fall.

Here is the quote taken from gouranga.com:

"As if Acclaim's Burnout and Activision's Spy Hunter weren't enough, a US mag has confirmed that the PS2's superb criminal-em-up Grand Theft Auto III will be car jacking and cop blasting its way to Gamecube in autumn. Choice!"

As soon as we get the official confirmation we will let you know!

source GameCube Europe