If you're trying to catch the free busses to the Marriott, then T2. How are you getting to LHR from LGW? If it's a combo of tubes or trains (ie, if you're on the Picadilly line), then as you exit the Pic line at LHR just head upstairs on escalator or elevator to the bus stands. If you're doing a bus/coach from LGW to LHR, they'll deposit you in the overall bus area as that's the staging area for private & public busses, so just go to the appropriate stand.

Cheers.