I run my gps off the 12V system on my bike.
For hiking, there are numerous aa and aaa solar chargers available, but pay close attention to the current flow. It could take weeks to charge 4 2900mah batts. One tip is to use the same batteries for all your outdoor electronics so you can swap in a pinch.
And try to use the GPS to get a location fix, then turn it off and use maps.