Yes, it is gas tight when used with the optional GL 45 EPDM gasket (1152752) for improved sealing. Unfortunately, this item is only available through the manufacturer. Please see the information and links below for additional information:
DURAN® Flat sealing O-ring Gasket GL 45, EPDM
Catalog Number: 1152752
https://www.dwk.com/duran-flat-sealing-o-ring-gasket-gl-45-epdm-1152752
It is recommended to use the 3-port screw cap for the specified need of 2 ports for passing tubes and 1 port for injection:
Screw cap GL 45, PP, 3 ports GL 14
Catalogue Number: 1129751
https://www.sigmaaldrich.com/product/aldrich/z680400