Six and a half hours is more than enough time. We used to have to do the Gatwick-Heathrow transfer every time (the British Airways Denver-to-London nonstop used to fly into Gatwick), and I've done it the other way, too, and it doesn't take nearly that long to deplane, go thru immigration/customs, wait for your luggage at the bag carousel (except in the unlikely event your bag doesn't show up because it was lost or misdirected), make your way to Gatwick, and check in there. We always used the handy, fairly frequent bus that goes between the 2 airports, but it's been a while, so I have no idea what the bus costs now. Don't dawdle at Heathrow, just so nothing slows you down, but you won't have to be in any big rush. Having carry-ons guards against lost or damaged luggage, but can limit what you bring, and checking bags when you check in in Dallas won't prevent you from making your Gatwick flight.