What component is represented by reference designator GT?
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.