Yes gravy can be reheated.
I reheat gravy by adding the gravy to a pot and then adding a bit of water to the gravy and then turning the stove on to a low temperature to simmer it.
Then after a few minutes the gravy is reheated.
Or you can also add the gravy to a microwave safe bowl and reheat the gravy in the microwave but the gravy may explode in the microwave so I find it best to use the stove a pot to reheat the gravy in.
Just stir the gravy as you're heating it back up on the stove and watch it so that it doesn't burn.
I freeze homemade gravy in Ziploc bags and then put the frozen gravy directly into the pot and then it thaws out and heats back up.