thankx to amjadmalik75 missing are now onlky two stadards:

CEN/TS 15399:2007 Gas Supply Systems - Guidelines for Management systems for Gas Distribution Network
EN 16726:2015 Gas infrastructure - Quality of gas - Group H


anyway, fresh links:

[link Point to another website Only the registered members can access]
or

[link Point to another website Only the registered members can access]