I wish I could answer that but I've never played gran turismo. If your new to the FORZA franchise I'd recommend you pick up Forza 3 first. You can get the collectors edition with all the dlc for under $20. Well worth it. Same with FORZA 4. 3&4 are more traditional racing while horizon is more free roaming. The online community is huge as well.
Did you play Forza 3 and/or 4 with a wheel? I have two wheels and neither of them are compatible with Xbox. It really sucks that I'd have to get yet ANOTHER wheel