Bitnix:
1) If you take a peek in your settings on Gmail.com ("Forwarding and POP"), you will see that GMail provides with guide on how to "intergrate" GMail in Mail.app. 2) If you have Leopard it should already be built-in in Mail.app (just make a new account)

Is there any similar app like this, that can handle Gmail in your Mail.app? (I.e. Mailplane, but integrated into Mail.app instead.)

I've been looking for a way to use Mail to get my Hotmail messages and at last this app has made it very easy to set up.