In case it was not clear. You can get and use a GPS only Apple Watch, and it will do Everything the Cellular model can do AS LONG AS your iPhone is nearby, where nearby is generally somewhere in the same moderately sized home. The Cellular model allows you to leave your iPhone at home while you go for a walk, run, bike ride.
The GPS only version does not care about your cellular carrier, and your cellular carrier does not care about the GPS only Apple Watch. The Apple Watch just uses your iPhone for all cellular services, and your iPhone is the one with a contract. In this case, just think of the Apple Watch as an advanced Bluetooth headset.