haha ever the assessor, you can only run gas through pipes that are "gas pipes", who fitted the original pipe? was it a gas fitter, if not then you cant use it, (i know it has been under 1-1.5 bar, and gas is only under 30mb) but nevertheless it hasnt been fitted by a gas engineer then you cant use it, who amongst us would connect to a pipe fitted by A.N.Other and when the brown stuff hit the fan, expect to get away with saying " I thought it would be OK", yes we connect to existing gas pipes without a 100% inspection on the existing run, but a visual inspection on a best endeavour and a TT is all that we can do, but on an existing known water pipe there could be a suitable blank cap, but not suitable for gas that was missed as a full inspection was not seen, and also compression fittings could be sealed with inappropriate jointing material for gas etc etc etc
TBH on a more serious note, it cant be a good idea to run gas thro' a manky pipe it will cause problems at the gas valve sooner or later