I’m not a Garmin User, but if you have an iphone that might be a good alternative. On a recent trip to Ecuador I used my IPhone’s GPS to keep track of waypoints and pre-established tracks. I used the application MotionX-GPS, which uses openstreet and opencycle maps. Just make sure to download the appropriate areas you are planning to visit before you head out. The application is just a few dollars and the maps are free. If you are going for several days without access to a power source, you may need to carry a small solar charger to recharge your iphone. I use the GoalZero’s Nomad 7, but I know they have come out with a smaller / lighter version since I bought mine.
p.s When are you planning to go to Peru. I'm also going, planning to do Huayhuash circuit, probably July or early august.