The GNSS (GPS) FMC on the 75/76 is called Pegasus, and as someone else noted, was introduced in the mid 90s.
I can't speak for other airlines, but as of today, AA's 75/76s are about half Pegasus and half Basic (IRU/DME). However, there is a time frame - I'm not sure of the exact date - where IRU navigation will become obsolete, and everyone must transition to GPS.
AA has been installing the Flat Panel Display System on our 75/76 fleet, which looks much like the 777 panels (albeit a much cheaper version).. and along with that, they are replacing the Basic FMCs with Pegasus.
Any 75/76 that AA received past 1998 or so came new with Pegasus FMC already installed.