I went both north and south through the gap last year pulling a 6500 pound TT with a 3.5 Ecoboost F-150. Going north, up that is, was effortless--65 mph in the left lane. Truck was in 4th gear running about 2500 rpm. No sweat.
Going south, down, was also easy--came down in tow-haul mode and tapped the brakes only twice, once because traffic had slowed down.