They fight in NYC
They were similar in strength based on tossing a city bus about. Kong a bit smaller but more nimble. He's tangled with enough giant monsters, I would be inclined to give him a slight edge.
It's totally conceivable Gargantos smothers him with his bulk and restricts his limbs a few times out of 10 but I can also see Kong playing safe and striking his eye when the option shows up.
"At the end of the day, Arby is a pretty prolific poster proposing a plurality of proper posts for us."
- big_adventure
It might be difficult for Kong to defeat an invisible opponent.