Section #5.1.10.11.5.8: Holds the answer !
( even though its seems crazy in this case )
The wording is - "SHALL CLEARLY IDENTIFY THE PIPELINE BY SPECIFIC NAME".
In my opinion, that's each gas by names or chemical symbol and color coding.
This type product / tape is readily available from many suppliers.
Now, lets talk about that marker list in Section #5.1.10.11.5.9:
Yes the one half the depth of burial.
This continuous warning means, could easily be debated as a single marker / tape.
(i.e. a single tape with the wording medical gases buried below)
All section from NFPA 99 2015 Edition