You're mixing up grounding and bonding.
Grounding is primarily provided by the ground rods, and is used to dissipate natural sources of electricity (lightning, static electricity) to the earth. This can potentially be done by a single ground rod, but how well it works depends on the local soil conditions -- if you want to use a single rod, you need to test and make sure that single rod provides a less than 25 ohm connection to the soil. If you add a second rod, you're pretty much guaranteed to get under 25 ohms in most soil conditions (or maybe not, per Ed Beal's comment), so the NEC does not require you to test in that case. Since the test (either buying the tester or hiring someone who has one to do it) is quite a lot more expensive than just putting a second ground rod in, most people opt for two rods.
In addition to that, your other plumbing must be bonded to your electrical system's ground. This is so that your pipes and the housings of your grounded appliances are always at the same potential, and there's no chance you'll be shocked when you touch your sink and fridge at the same time.
There used to be a provision in code for using your plumbing systems instead of one of the ground rods, but that dates back from the time when water pipes were always metal. Now that plastic water lines are becoming more common, using them instead of a ground rod on a new installation is no longer allowed in most areas, but they must still be bonded.
So in short, two ground rods, plus bonding to both the water and gas systems, is the easiest-to-meet requirement. You can possibly get away with one ground rod + bonding to water and gas, but to do that you'll have to do an expensive test, and depending on your soil, you might have to install the second one anyway.