My primary hiking GPS has topo maps built in to it, but you don't need them for general route-finding. Even the most basic of GPS's will plot your path as you travel and that way even if you feel lost, all you do is follow your "breadcrumbs" (in this case, on the GPS) back to your starting point. I usually make a waypoint for the trailhead I begin at, or my car. A waypoint is just telling the GPS "Hey, take a reading right now and save it." You usually get to name the waypoint, so it'd be something like "Start" or "Car" or "Campsite" or whatever. Most GPS units have a "Go To/Route To" function so once you have that starting point, anytime you want to return you'd call up that function and tell the GPS to start routing you back to it.
That's a real basic description. I generally don't carry topo maps, but that's because I'm not backpacking and I'm familiar with the trails I'm on (more or less). I've used the GPS to scout around areas and "map" trails that are informal so I have a visual representation of where they actually go -- even a basic GPS with no topo/good maps, with the track data and the right software (some of which is freeware/shareware downloadable online) you can correlate your track to maps after the fact.
Guadalupe's trails have a few places you could get slightly turned around, but there's minimal getting lost potential. I say get a cheap GPS, turn it on and let it acquire signal, and start walking. Soon you'll have a mapped trail of where you've been and you can wander with the knowledge you know how to get back where you started. Bear in mind in tree cover the signal accuracy drops slightly, and acquisition time increases, so they generally get you in the vicinity of where you want to go but not right on top of it (hence the popularity of geocaching -- there's still some hunting and scouting to be done for the caches that are hidden).
I'll answer more when my late lunch isn't getting cold on my desk.
