If you are connecting onto it then yes you will be made responsible in future if anything goes wrong as you will be certifying the complete system (including the underground pipe). This means that you the Certifying Gas Fitter are taking responsibility for the complete installation.
I'm not sure if getting your client to sign a document excluding you from any responsibility on that gas pipe is actually leagal, its like getting a costumer to sign a document excluding a plumber to take responsibility for an installation for not installing a tempering valve because the client didn't want to pay for it. Then something happens... (we have to install a tempering even if there was no tempering valve before replacing the HWC)
I believe you need to advise your client that certain tests need to be made ("charge up" of course
) to ensure the pipe is installed correctly and safe. Tests such as a pressure test or even a camera down the pipe (you can messure the level from the ground as the camera is pushed through) to determine whether its laid straight (no dodgy bends etc) and the internal condition will give a more confident feeling before signing the dotted line.
In the climate our industry is in at the moment you have to be very careful. Hope this helps.