GPS requires a reliable mobile network connection for the tracking purpose. The GPS cannot be confined to a single network as it is not immobile and moves around different locations gathering data. If it looses the connection then this gathered/tracking data cannot be transmitted to the tracking platform/software. In order to know the exact location of the tracker, a sim card is inserted just like in our mobile phones which will use the cellular network to send the gathered data to the server.
Usually, the Machine to Machine a.k.a M2M sim cards are employed for this purpose as they showcases better performance and are cost effective.