Another option could be to go with an AAA powered GPS, IIRC I saw a 3 AAA powered last time I was looking through web sites for them.
I'm going to go with an AA powered as everything in my large bob is AA powered, flashlights, radio, scanner, etc. I'm down to AAA's in a radio in my truck bob and my Arc AAA EDC, and a couple AAA powered Mag Lights in the truck bob, all of which are I plan to rotate out to AA models. Primary power is Sanyo Eneloop with Alkaline and some Lithium AA's for back up.