The GPS receiver in your iPad (and your phone) works in real time and it works independent of cellular signal, so it works fine off grid.
But your iPad (and phone) app is set up to pull maps for whatever you're looking at from the Internet, meaning it needs cell/wi-fi signal to render a map on the screen. If you were off grid will no cell signal and no downloaded maps, when you opened the app on the iPad you'd see a gray screen with a blue dot in the middle. The blue dot is you, showing you that the GPS receiver is working. You just can't see where you are because the app can't render a map around you.
That's why you download maps. Once you download maps from the Internet to your device (iPad or phone), then it no longer needs the internet - it has the maps on its internal hard drive. In fact, when I'm in the woods I put my phone into airplane mode to save battery, since I no longer need cell signal. So simply download the maps to the hard drive of the iPad for the area where you intend to be hunting and everything should work fine. I would recommend using airplane mode when you're out there to save iPad battery.
This will happen to you, because it happens to all of us at least once: You'll forget to download the relevant maps before leaving home. You'll then spend time in the parking lot of a coffee shop or whatever logging into a public wi-fi trying to download the maps before entering the woods. Download while you're still home!