I also use Gaia with an iPad. It’s probably best to have a cellular capable iPad for the internal GPS, but if you happen to have a WiFi only iPad, you can use a Bluetooth enabled external receiver like a Garmin Glo2.+1 for Gaia again on your tablet (With GPS—need cellular iPad but dont need cell service or cell plan) /phone connected with car play.
The upside to using a Bluetooth GPS receiver with an iPad is that you can position it for better reception while keeping the tablet where you want it and, unlike the internal iPad GPS, many of them are WAAS enabled. Of course, you get the hassle of a separate device to mount, charge, etc and the antenna is still not as good as a quality, dedicated unit....The GPS antennae’s in these devices kind of suck.