To my mind a supplemental ground is like another ground rod and such.
Bonding the black pipe provides no additional path to the earth worth mention.
The local gas company delivers the gas via buried plastic. Then it is connected to an extremely short stub of black pipe on the way to the gas SERVICE.
With our dry soils and climate... No one would consider that short path any kind of supplemental.
Every additional element brought into the GEC System does so by being bonded to it to create/extend an equipotential plane.
Connecting the neutral jumper causes the grounded conductor to be --- grounded.
All bonding connections are for fault conditions only/maintaining an equipotential plane.
Grounded conductors and their path down into the bowels of the GEC are expected to carry the return current/unbalanced current.
That's my frame of reference...
SERVICE bonding is typically 3/0 Copper -- by specification.
However, from time to time Aluminum has been used for some bonding connections when the ENTIRE run is above ground and conditions are suitable. ( Would never install near the sea coast, etc.)
Again, I only work commercial so some of my practices may not line up with residential practices. I do normally get inspected by the 'top dog' in the area. For example in downtown Oakland all of the high rises are inspected only by the City's Chief Inspector. He doesn't miss a thing.
I must say that in residential settings our AHJ just won't allow aluminum, period, in field wiring/bonding. The Poco loves it though. If your SERVICE is so huge that Copper is required ( over 3,000 Amps 208V or 480V ) you're tapped big time and the stuff is brought out just for you. It is NOT stocked. Normally SERVICES of that size move to bus duct and put the Poco Xfmr immediately outside the building.
And in my area the Poco is the Gasco.
