A lot would depend on how you wanted it to look. You could remove the grip and clean up the chip area, then glue a small piece of wood that matched, as close as possible, and then shape it to original dimensions and finish.
Or, take the grip off, rough up the broken area, tape the grip frame and reinstall the grip. Mix up a batch of JB Weld and fill the void. Once it sets up, remove the grip, shape the grip, remove the tape from the grip and reinstall.
OR, just buy a new set of grips, you can probably find them relatively cheaply, depending on the model of the gun and what the grips are made of.
Oh, by the way, you did not tell us what gun or what the grips are made of, that could make a big difference.