Yes, it is possible. In fanfiction, writers have the creative freedom to reimagine the origin of characters. Goku being born with Ultra Instinct can add a whole new layer to his character development. It could mean that his power growth would be different from the original story, perhaps faster or with different challenges along the way.
You could write a fanfiction where Goku teaches others about Ultra Instinct. Maybe he tries to pass on this power to Gohan or Trunks. This would involve Goku having to understand the power more deeply himself in order to teach it, and it would also be a great way to develop the other characters.
Goku definitely masters Ultra Instinct in the manga. This transformation gives him a huge boost in power and allows him to take on even stronger opponents.
In the fanfiction world where Goku gets Ultra Instinct against Zamasu, the battle is a spectacle. Ultra Instinct Goku is like a different being. His reflexes are on a whole new level. He can sense Zamasu's every move before it even happens. Goku might start by testing Zamasu's defenses with quick jabs. As the fight progresses, he uses his Ultra Instinct to launch a full - scale offensive. His attacks are not only strong but also strategic. He might target Zamasu's weak points that he couldn't reach before. The fight could also involve Goku using Ultra Instinct to break through Zamasu's time - based abilities if the fanfiction has such elements.
Often, Goku with Ultra Instinct is more focused. He's not as carefree as usual. He's fully concentrated on defeating Zamasu. His determination is palpable.