Loading gas into this gun is awkward.
Tighten the brass screw as much as you can before operating the heel lever.
If the gas doesnt load release the heel lever and tighten the brass screw again and operate the heel lever again.
It is easy to turn the heel lever the wrong way and get out of sync. causing all sorts of hoo hah.
If you are doing it right but the gas is still leaking put some pellgun oil on the tip of a capsule and fire it through.
If the problem persists the seal may have gone.