What makes you think that GPS could decrease the needed separation?

Because that is exactly what it is designed to do?

It _may_ be able to more precisely control separation out in the airways,
and get them set up for landing sequence, but notice I said "may." They do
a pretty good job with radar, right now.

What it _can not_ do is put more aircraft on the runways per hour in the big
airports operating with all of the landing slots full. The separation for
wake turbulence is always going to be the limiting factor in how many
aircraft can land at a given busy airport at peak times. GPS is not going
to change that.
--
Jim in NC