Well, the forum is separate from the GCU site in that it is 'run' by a forum group set up through GCU admin. That is why when you are logged into GCU working on your store, you still have to log in to reply to a thread on the forum.
But, you don't have to log into the forum to log into your store. When you bring up the GCU homepage, if you have told it to remember you, you will see upper right corner of the screen a Welcome YOURNAME with a LOGOUT next to it, meaning that you will see a Manage Store link up there and you are already logged in.
If you bring up their homepage and are not logged in just login using that same link up on right side of the screen.
Does that help you in anyway