That gasket is called an *exhaust* gasket for a reason you know...
If the gasket isnt in place, exhaust will seep thru the head and around the pipe. In other words yer gonna have a second exhaust pipe pointing straight out the front.
Its not nice to have exhaust pouring out of your cylinder head, believe me. Sure its not bad while going, but its kinda nice to let your bike idle a little while talking to buddies, checkin out the trail ahead, or any other reason.
just my .02