The only thing you do in Gears is run, shoot, and take cover and maybe there's one puzzle scene and about a two minute scene in the vehicle.
In Uncharted there is a crap load more to do and that accounts for the differences in the lengths of the games.
I played both and I know what I'm talking about it. And if you can't accept that well, you can always reflect on the two words that DX had for everyone.