Here's how I tested my GP's.
Ground one by laying it on a piece of steel; I used the tongue of my trailer. Attach positive battery charger lead to the top of the GP, where the wire would be attached. Attach the negative elsewhere on the steel, so current will flow through the GP to it. Plug in battery charger and see if the GP actually glows. It may take 10-15 seconds for it to start glowing. If that doesn't work, the GP is no good.
Good luck with it and let us know outcome.
Now thats some good tech. Ill give it a try while I wait for the new GP's to arrive.
Thanks