As you're a GCH, just check the map in MMB.
Some of the 3 class 777s have a row 5, some don't (someone who knows this stuff will probably be able to tell the difference)
I had 5K on a LAS trip recently and it was fine - extra legroom, window, direct aisle access and it had AVOD, but it broke as soon as we left LHR
(crew were rubbish, though, as was the food, but that doesn't depend on the aircrraft!!)