The building architect was not informed you were moving the gas line inside your apartment - ?

No. I had the plumber coordinate with the building superintendent who was present when the work was performed. Given the gravity of this error, I don't understand how the super didn't know about this issue either.

He got a tip - right?
Ours overlooks illegal stuff by contractors all the time. and get's away with it despite the problems that result. Really coops and condos should not involve Supers in significant renovations. I might add that since your Super knew about the situation you are entirely off the hook as he is a coop employee. His mistake = it is the coops mistake. Put it in writing to the board.

I don't believe he received or even wanted a tip. The gas valve was leaking, typical for an 85yr old valve. I think he honestly thought he was acting responsibly when he allowed it to be replaced.

I spoke to a real estate lawyer friend. He said the bldg riser is an existing condition. The alteration agreement holds me responsible for damages I cause. My plumber replaced a leaky valve. To the extent he damaged the branch line or the bldg riser while replacing the valve, I would be responsible. But he didn't damage anything. The plumber's work triggered a DOB rule that any work done on a gas valve requires a subsequent bldg riser pressure test. If the bldg riser was brand new, the riser would pass the test. But the pressure test will fail not because of anything my plumber did but because the bldg riser is 85 years old and already in need of replacement. The poor condition of NYC's gas pipes are well known in the popular news. There have been a few reported cases of people smelling gas and calling ConEd. The DOB/ConEd then come in, shut down the gas, and require the building to replace its riser. Those individuals are never held responsible for the cost of replacing the building riser. They acted responsibly but could not have anticipated the consequence of notifying ConEd. What if I had opened up my wall, discovered an illegal gas line and called ConEd. The same outcome (i.e. bldg riser replacement) would've occurred. I wouldn't be held responsible for replacing the bldg riser then, would I? My plumber did not cause any damage to the riser; did not touch the riser even. To be sure, he may be fined for doing work on a ConEd pipe without obtaining prior ConEd authorization. But that is a completely separate issue. A more complicated case arises had my plumber put a sledgehammer thru the building riser. Then the questions of quantifying the damage and splitting financial responsibility become more murky.

In the building alteration agreement, did it specify that no alterations are to be done to change the existing layouts of, and for example, gas, pipe, telecommunication etc. lines? If it did and you proceeded regardless, then you may be "on the hook". If it did not, then you are not responsible; it is up to the board of directors to ensure that shareholders understand the limits of renovations that is allowed. Normally there is a clause specifying what is allowed - and what is not.

The superintendent would only be responsible if it has been clearly communicated to him (and in writing) what he can and cannot approve i.e. "cannot approve any gas line work". However your licensed plumber should be aware of any restrictions of his trade (and is a licensed plumber "plumbing" the most qualified to do the work that was performed which could explain why he was unaware of DOB requirements?).

I would make myself available when the tests are done by DOB so that you can hear first hand whether or not the work you had performed impacted the pressure of the risers.