Most folks find they can restore good function by putting a piece of 20-gauge copper or brass wire at the joint, like an o-ring. This will seal the joint. I simply coated my gas valve (the inside piece) with Blue RTV Gasket Maker, and it has held up through many range sessions (several hundred rounds). I don't expect to use the grenade launcher, but this sealer remains soft and is easily removed.
Some with machining skill have made new valve-bodies to fit snugly in the housing. It's pretty simple to make on a lathe.
The fix depends on how bad the leakage is. Some aren't too bad, others are very corroded.