Our trackers absolutely need a GSM network to function. If you pass through a so-called 'dead zone' where the network doesn't reach, the tracker will have difficulty communicating its position. It will record its internal GPS points and transmit them again when it regains network coverage, but the internal memory is limited and only holds a few kilometers' worth of data.
You can also check the quality of coverage based on frequencies, operators, and the municipality you are traveling through.