Stephen, I've been talking with the folks at Laminar Research and the gates should be connected to taxiways. This is a "recommended" practice but not required. While it was acknowledged that the AI planes may not work correctly without the gate to taxiway connection, I guess they just didn't want to make it requirement. (though in my opinion they should) In any case, Laminar's KSEA scenery doesn't have the connections but it "should".
Justin will be adding the connections to the KSLC scenery when he does a v1.1 update. I am currently working with his scenery to put together a file for X-Life which will have all 160+ gates work correctly with that plugin.
Frank