A GPS tracker will still record location information through these areas of spotty coverage, however, and then report that history through the GPS platform when the device regains a data connection. Think of it like trying to send a text in a dead zone; the text will eventually send once cell service is restored. When you view the tracker's location history it may not indicate a dead zone, because it will be recording location information throughout; the dead zone will only present itself when you view the tracker's live location and it happens to be travelling through an area without coverage.
It's the nature of communications technology to change over time. As faster data connections become available, they replace older modes, yet still rely on existing infrastructure (towers) to transmit data. By shuttering 2G GSM and dedicating the same airwave space to 3G and LTE, cell providers like AT&T can increase data capacity by more than a hundredfold.