Sonic Unleashed is a different game than Sonic the Hedgehog, and it doesn't include classic stages like Green Hill Zone. It features brand new levels and locations, though it does have a nod to the classic game with the "Windy" level, which is reminiscent of Green Hill Zone.
If you're looking for Green Hill Zone, you'll need to play one of the earlier Sonic games, such as Sonic the Hedgehog (1991), Sonic the Hedgehog 2, or Sonic 3 & Knuckles.