When you mount it upside down, do not put the rear screw in the gas block. Just use the front screw (if it's a two screw setup). The rear screw hole is ALWAYS drilled directly opposite the gas port, and if you put that screw in, you'll undoubtedly damage the gas port in the barrel. You WILL have gas escaping. You could loc-tite a set screw in place, without cranking it down the barrel's gas port, but that gas is hot, that loc-tite will break down, and that set screw will eventually fly outta there like a rocket one of the times you fire.
Use a clamp-on gas block, and you won't be tempted. You will still have gas escaping.
Just thought about this - if you use a clamp-on, and mount it 90*, instead of directly upside down, you'll be able to seal the barrel's gas port that way.