I can see that G represents a generator/oscillator, and T represents a Transformer, but what does GT represent?
As I guessed in my comment, it's a Gas Discharge Tube. As Dave Tweed says, it's an overvoltage protection device, or more precisely, it protects against high voltage spikes as opposed to long term faults.
Here's an excerpt from a Littlefuse datasheet:
GDTs function as switches which dissipate a minimum amount of energy and therefore handle currents that far surpass other types of transient voltage protection. Their gas-filled, rugged ceramic metal construction make them well suited to adverse environments.