Backpacks: There are many good brands. Personal preference I choose Eberlestock
for the built in rifle scabbards. I have three. The J107 (J34) is really larger than you need for a day pack. For a true day pack, the X1 variants should work if you want the scabbard, X2's if not. This year I am trying out an X3 with a little more capacity and molle webbing on the outside if I want to attach other pockets, pouches, etc.
GPS units: Again personal preference. I use a DeLorme PN60. Not the cheapest. Not the simplest to learn. But, good reception, and good maps. Good detail on the USFS trails, even when I have to look to make out the trail. I tend to solo day hunt in Colorado. Go out one way. Come home a different way. Have always made it back to the cabin each night.
Now, if I could just get the GPS unit to show me where the elk are....