Hi Kiwitan,
Most rcr123's are rated at 3.6v, and come off the charger (max) at around 4.2v each so they would hotflash/ruin the P60 bulb in your G2 as it is rated for 6v, and you would have 8.4v between the 2 freshly charged batteries. There are some rechargeable 123's that are rated at 3v, but from what I understand they come "hot" off the charger too.
Now, if you really want to go rechargeable in your G2, you can use a Surefire (or other brand) P90 lamp, or Wolf-Eyes D26 9v lamp, or Digilight 9v lamp with two of those 3.6v batteries. This combination puts out a LOT more light. Please note that the "unprotected" batteries are the only ones I have and can say will definitely work 100% but you have BE SURE not to drain these batteries or they could overheat, leak, "rapidly vent with flame." So if you do this, charge the batteries as soon as you see the light dimming. I don't know if protected cells will work in this setup but maybe someone else here does?