In a 6J5 with no letters , the pin marked “m” is connected to the metal case. (check it with an ohmmeter if you have a metal 6J5. In some tubes S or IS is a connection to an internal shield.
A 6SQ7 has in effect, 3 plates. 2 act as diodes and are labelled PD1 and PD2. The other P is the plate for the triode and is labelled PT.
I can’t help you with “GT” other than to suggest it is leftover from something else.
In a tube with a triode and a pentode such as a 6AU8, the grid of the triode is labelled GT and the grids of the pentode as g#P.
There is no real requirement for it to be labelled GT as opposed to just G. in a single triode tube such as a 6SQ7.
Just my 2 cents. Perhaps there is a better reason out there .
John G. VO1 CAT