How do you play as Silver in Sonic Adventure DX for GameCube?

You can't play as Silver in Sonic Adventure DX for GameCube. Silver is a character from Sonic the Hedgehog (2006) and later games. He wasn't introduced until after Sonic Adventure DX was released.

Sonic Adventure DX features the following playable characters:

* Sonic the Hedgehog: The blue blur himself.

* Tails: The two-tailed fox.

* Knuckles the Echidna: The red echidna.

* Amy Rose: The pink hedgehog.

* Big the Cat: The large cat.

* E-102 Gamma: The robotic badnik.

If you're looking for a game where you can play as Silver, you'll need to try Sonic the Hedgehog (2006), Sonic Unleashed, Sonic Generations, or other more recent games in the series.