gastightness

gas·tight

 (găs′tīt′)
adj.
Impermeable by gas.

gas′tight′ness n.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

gastightness

(ˈɡæsˌtaɪtnəs)
n
the state or quality of being gastight
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014