The Collaborative International Dictionary of English v.0.48:

Gastight \Gas"tight`\, a.
   So tightly fitted as to preclude the escape of gas;
   impervious to gas.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gas-tight
    adj 1: not allowing air or gas to pass in or out [syn:
           airtight, air-tight, gas-tight]