We currently offer a massive range of downloads for FSX, as well as older flight simulator add-ons for the ever-popular FS2004, FS2002, CFS3 and now becoming ever popular with dediced virtual aviators is Laminar Research's X-Plane series.
Go through the learning center and learn how to land manually first.
You cannot (as far as Im aware) actually land using gps. You can use GPS to navigate lateraly, but it will not change your altitude. I think what you are looking for is landing using the ILS.
To land by ILS, you must program the ILS frequency into your radio stack and activate the Nav switch for whichever radio you programmed the frequency into. Set you course to the runway heading. Fly your aircraft to intercept the beacon approx 10 miles off the ILS tail (green arrow on map). Set your GPS / Nav switch to Nav, set your autopilot to you current heading (to intercept beacon), activate the APR autopilot. When you get close enough the autopilot will take control; the HDG autopilot switch will turn off, the APR is now active. The autopilot will turn the plane into the direction of the ILS and begin to line the plane up. Ensure you know the altitude of the runway. Depending on the plane / class of ILS, you may have to negotiate the descent yourself; otherwise the autopilot will descend to the runway. Follow the glide slope to the runway. When you are around 500 - 1000 feet up, turn off the autopilot to land the plane manually; unless the aircraft you are in has auto-land, you still must land manually (if you dont want to touchdown at -500ft/min). When you do turn off the autopilot to land, get ready to counter any cross winds etc.
RE: How do I find the ILS frequency and my runway heading?
just tune your NAV1 radio to the ILS frequency of the runway at which you'd like to land. Be within ~30 degrees of the runway heading and ~2,000 ft above the runway elevation (be sure to make note of that when you get the ILS frequency) and trimmed for and at approach speed well in advance. Turn on APP and the autopilot will do the rest. If it's just an ILS approach you want you do not need to worry about "VOR stuff". You will be responsible for flaps, speed control, speed brakes (if required), disengaging the autopilot prior to touchdown and flare. I do believe that's the most brief explanation I've ever given of an ILS approach :-) - Good Luck