I dont know about all airlines, however i've heard of Horizon using GPS for some approaches in Canada, and the north west. Obviously some of the airports they fly to are quite remote and nestled nicely in between mountain peaks. I also spoke to an old instructor at pinnacle who mentioned the FMC and GPS usage during approaches. Its safe to say when our turn comes around we'll be using primarily GPS for almost every portion of the flight. Some interesting things being done with Naverus, a new company out of seattle that specializes in developing RNP GPS approaches. Take a look at them to understand some GPS capabilities