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I can see that G represents a generator/oscillator, and T represents a Transformer, but what does GT represent?

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An oscillating transformer?: I can see that you can't see that we can't see what you're talking about without some more information. How about editing your question so we won't have to guess/close your question? – EM Fields Jan 13 at 13:54
It could be something like a gas discharge tube, but it's hard to guess. Do you have a picture of the component? – Armandas Jan 13 at 13:55
"Gas Tube" -- it's an overvoltage protection device. – Dave Tweed Jan 13 at 14:05
Designators can be somewhat arbitary. There are conventions for the common stuff but when one has an unusual component a designer will often end up making it up as they go along. – Peter Green Jan 13 at 15:16
@Roy If my answer helped you, please consider accepting it. Thanks. – Armandas Jan 15 at 18:16

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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.

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