I'm an A/C engineer by trade, domestic/commercial, not automotive, all I know is they use R134a.
There's a few things you can check for a full charge of refrigerant.
The air temperature difference (between air on and air off the coil) should be around 10-15degC. e.g outside temp (or cabin temp if in re-circ mode) 20degC, air off should be between 5-10degC. This will differ from type of system/refrigerant type etc etc.
There is also a nickname/term we use called "sweating back" which indicates a full charge of gas, without the use of/attachment of gauges.
Whilst in cooling mode, the suction pipe from evaporator to compressor (larger diameter pipe) will be ice cold, will condensate and form water on its surface, (depending on RH) but not freeze/form frost.
Not sure where the suction pipe is exactly on the GTD, will check this later for you.
