You don't need either cellular or a built in GPS. What you do need is a really bright screen, and Bluetooth. Then you can hotspot from your phone to the tablet's WiFi, and put your GPS somewhere convenient with a good sky view. I find seven or eight inch tablets running Android work fine. Don't add anything beyond what you need to make the GPS work.
or you can just put your phone on an A5 kneepad, but the external GPS is still a good idea. You can add a battery pack if you want, the GPS should run longer than your flight will last.