To be clear we are required to use a metal water line as an electrode as long as there is 10' of it in direct contact with the earth. It does not matter what is beyond that 10'. An 8' rod is an electrode so it only follows that 10' of metal water line in the ground is an electrode. It will not matter if that 10' of metal line is supplied by non-conductive mains.
In the big picture it may be better if you water line was not electrically continuous with your neighbors.
Yes we must supplement it with one or sometimes two additional electrodes. My guess is that is required as many times people disconnect the GEC from the water line and do not put it back.
I will have to look but I do not believe there is a minimum size bonding conductor for a gas line.
