WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Don't have a specification, but it would be a low value. It's designed to work with positive grid voltages.
The similarly-scaled pentode, the GK71, is rated to have a Maximum grid leakage current of 30uA, when running 83 .. 100mA in the anode - I would imagine one should work with at least that much leakage, and gas will add to it, because of the years of storage.