gps, nope there are good cell's out there with that info other then topo maps. but some have maps you can buy I understand.
rain gear, for sure! rather be comfortable and fish when you want then be limited to when you can fish.
gopro? I have one and didn't use it that much at all cause I didn't have a chest harnes (but I do now, thanks to Will! ) so I may use mine more.
Knipix cutters are a great tool to have in case you hook into a big'en and need to cut it loose or free a hook from your hand or other parts. they cut through hoohs like a hot knife through butter.
other ideas... minnow bucket with a pump and you can always use a good spool of line too.