I think stuffing 7 billion people into that valley would be just a tad too crowded. It would be more logistically practical to simply commit species suicide.
It'd probably be less crowded than your typical airline flight.
Well the floor area of the gorge is about 48km by 20m, so I estimate that if all 8 billion humans were sharing that space, they would have about 120 square millimeters each (that's about 0.2 sq in per person for those watching in black and white).
So only a little less crowded than coach class on a low-cost airliner.
Economy, bilby.