Trick, I think you are confusing "bonding" with "grounding."
This is also an area that is usually modified by your local codes, so you will want to check thing out at city hall, as well.
The wire that goes to your ground rod....the "grounding electrode conductor"....is specified by the NEC to need be no larger than #6, without regard to the size of the service. Many places have raised this to a minimum of #4 solid, and further require it be in pipe for extra protection.
Metal parts of the house that are likely to become energised are required to be "bonded" to the system. Such thing include the water line...but the gas service again is an area that local codes differ about. In any event, the NEC specifies a #10 bonding jumper, but many places specify at least a #8 wire be used.
You are no longer allowed to use the water service as a "ground;" you need that ground rod. You are, howerver, required to "bond" the water line.
