Okay, we need to get a couple of things established. Forwarding your email implies you're getting your GMail account to send your emails on to a second email account somewhere. If this is the case then you should still be able to use that second account in both apps but setup will depend on what mechanism your account has - POP3 or IMAP or something else.
Now, assuming there is no second email account, then we can start talking about getting GMail to download messages to both Mail
and Entourage. GMail (to my understanding) has POP3 access which means you can put the details into both apps. However, you must ensure they are both set to not remove messages from the server immediately. When I had a POP3 account on two machines (effectively two apps), I had one machine
never delete the messages from the server and the other machine would delete them after a week. Assuming I used both machines in a week, this meant all new messages were downloaded to each machine/app. Of course, sent messages were not kept synchronised.
So, sorry for the long bits, but in short we need to know if you want GMail access from both apps or if you want to forward GMail emails to another account which can be accessed from both apps.
