Your leak is most likely caused by the poppet seal, I have experienced one or two random poppet leaks and there seems to be no apparent reason for them, usually they go away after 10 or 20 shots and arent a problem. If you take your poppet out and give the seal a wipe and also make sure where the poppet seals in the body is clean. It shouldnt be lubed though just give it a good dry clean, you should give the oring that is on the poppet a good lube though.
If it continues to give you trouble you may find that running your gat at a slightly higher operating pressure might cure it, you may have to turn your dwel down a few blips or simply adjust your velocity adjuster a bit to retain legal velocity.
if all else fails get a new poppet, if a new poppet doesnt work then the poppet seat is fudged/scratched and you may need a new poppet housing/bolt guide (or whatever its called) but its unlikey.