All GPS trackers need a way to report their location to you. That's done using mobile data networks - the same sort your phone uses. Most GPS trackers have companion subscriptions to make it simple, but you could look for a GPS tracker that is SIM-free and pair it with a data SIM of your choosing, such as this one from Three that has a set amount of data that will last for up to three months. This means you can avoid the monthly recurring fees but have the GPS coverage whenever you’re travelling.