As this is one of the few relevant results that comes up when Googling FEG GPM 01 I'm adding a bit of info.
I've just resealed an 01 CO2 valve using a washer cut from a CO2 capsule seal my local RFD dug out of his pistol spares boxes.
I still have to go back & pay for it & find out what pistol it was from, but if you have one of these that leaks don't give up hope
ATB,
Nick
I've now found a probe O ring from a local bearing & seal supplier so I've got the pistol fully working ;-)