WRITTEN BY RIYA DUBEY
This is a very difficult yet interesting question. Before fifth gear was introduced, ultra instinct was considered the pinnacle of fighting form.
According to what fans elieve, gear five is stronger than the ultra instinct. The two powers are very different from each other but are extremely powerful.
Goku's ultra instinct allows him to tap into his full martial artist potential. This form resonates with Goku's history as a martial artist.
Gear 5 on the other hand, gives freedom to Luffy. It is the closest form to Luffy's actual personality.
In addition to that, Ultra Instinct can easily allow Goku to destroy planets if not galaxies.
Whilst something like this hasn't been proven for Gear Fifth, it is proven that it can alter reality.
It allowed Luffy to conjure a pair of glasses out of nothing. What Luffy can do in this form is only limited by his own imagination.
Therefore Luffy can defeat Goku in his ultra-instinct form if he truly believes it and fights him at his full power. However, this answer may differ in the future