I will answer this as best I can as I have to go out in a couple of mins so here is a brief explanation.

Ive heard in the last few years that airliners have switched over to GPS navigation instruments,and Id like to hear from some of the pilots here what that has added in navigation capability.

Well not quite instruments but GPS updates our FMS. It is very accurate at navigating, so much so when we are in the cruise we sometimes put a one mile offset in to the FMS which makes the plane fly one mile off the FMS course, just in case we have another aircraft at the same level then we would miss it by a mile. Just as a back up.

For example I hear its possible to take off and land the plane with zero visability, entirely by instruments.

This has been available for many years, it’s more of an autopilot capability than GPS for landing. For the take off, you still need a certain amount of vis as you need to steer the aircraft.

Has the new GPS navigation taken over traditional instruments like altimeter, airspeed indicater, artificial horizon, rate of climb indicater, etc?

No, most of the instruments in the 747-400 are fed info from the Air Data Computer.

Like would new “glass” cockpits have instrument readings all fed from GPS?

Again, the ADC.

Hope this helps.