I really dislike the FS default GPS. Its such a bad implementation of a "GPS". If you really want to use GPS, I would highly recommend the Reality XP units. A true representation of the GNS430 and 530 with WAAS and LPV approaches. Plus the NAV/GPS switch is right on the unit. Much more up to date as well, has SID's and STAR's, and you can change your flight plan on the fly, or go direct waypoint without wiping everything out. Many developers are starting to allow seemless integration of Reality XP into their aircraft for people that own it.

I've been using either CIVA INS on mine or just VOR/NDB. Sometimes I'll fly VOR/NDB and use the INS to fly an RNAV STAR. I also have the ISG gauges but haven't tried using them because I have FMC on too many other planes.