If the muffler has a gasket, leave it out...Check to see that the cylinder surface and the muffler surface are both flat..Get some Permatex Ultra Copper silicone sealer from an auto supply...Clean the threads on the bolts and in the cylinder...Put a thin layer of the Ultra Copper on the muffler and some Green Loctite 609 or 680 on the threads..Tighten securely...
The Loctite takes 400 degrees F to loosen, if your cylinder is getting hotter than that something else is wrong....The Ultra Copper is good to more than than 400, so it won't leak...
The muffler on a G38 is thin steel and is in two parts...Ultra Copper on the joint has not melted out or leaked yet...Same for the joint at the cylinder and muffler....No gasket...