Gruul Ragebeast's triggered ability isn't optional. However, if no opponent controls a creature when a creature enters under your control, the ability will be removed from the stack.

If one or both of the creatures that are supposed to fight are no longer on the battlefield when the ability resolves, the fight won't happen. Neither creature will deal or be dealt damage.

If the target creature is an illegal target when the ability tries to resolve, it won't resolve and none of its effects will happen. Neither creature will deal or be dealt damage.