Are two 8' grounding rods and clamps on the gas and water service necessary for 25 ohms?
Yes the additional water and gas lines need to be connected but it is not necessarily for grounding resistance.
The gas meter is grounded not to be part of the grounding electrode system but because the internal pipes may become energized. Code actually requires this.
The water pipes, if not part of the grounding electrode system, may also become energized thus need to be connected to the system.
You may find out the water pipes and your 2 electrodes do not get you down to 25 ohms. When I first started taking measurements, I expected very low values with water pipes and ground rods, but found quite often the values were above 25 ohms. In fact, many times I saw values up to 4x or close to 100 ohms, unless I connected the well casing or an Ufer ground.
So the more grounding you have the better:

25 ohms =4.8 amps to ground
15 amps to ground 8 ohms
20 amps to ground 6 ohms

The max voltage to ground in residential with split-phase 240v service
is 120v to ground so you can see to clear a 20 amp breaker your grounding resistance for a fault would need to be below 6 ohms.
So it is code if metallic water pipes and gas lines for them to be bonded to your electrode system.
The best grounds turn out to be Ufer or concrete encased and water pipe (well casings are great but only require bonding if there is a pump).
Some older homes have disconnected from their wells for city water hook ups.  Your 2 ea 8’ rods separated by at least 6’ are the minimum code allowed.