The Collaborative International Dictionary
Gastight
Gastight \Gas"tight`\, a. So tightly fitted as to preclude the escape of gas; impervious to gas.
Wiktionary
gastight
a. impermeable to gases
Usage examples of "gastight".
Upon firing, the hot gases pressed against the inside of the tube, expanding it to firmly grip the walls of the chamber with a gastight seal.
The gastight bulkhead would prevent any leakage, but depending on how much coolant ran off the reactor during descent, it could give off a lethal dose of radiation to the operators standing in the compartment just below.