If you're going to be re-introducing gravel that has been in 'dry dock' for quite a while, your best option is to simply give it a few minutes in boiling water, and then rinse thoroughly. This kills any:
- Organics (from insects, rodents, spores, etc)
- Anything and everything else
It'll also help to loosen up any dirt or debris that you wouldn't want slowly released in the tank after being submerged again. It basically brings the gravel back to the condition that it was in when you first took it out of the bag.
Don't use any detergents or chemicals, if your tap water is high on the chlorinated side then consider boiling and rinsing with distilled instead.
Just make sure that you rinse it thoroughly after boiling, and allow it to return completely to room temperature prior to using it again. Once back in the tank, it can begin accumulating the beneficial stuff, at which point you'd use a gravel vac to remove the unwanted particles that tend to accumulate at the bottom of the tank.