"Gotten" here means "arrived". The verb "get" has thousands of meanings, which makes it a confusing verb with unlimited uses. I think it would be possible to speak English knowing only three verbs: "have", "be", and "get", but I don't recommend doing that.
So the sentence fragment really means "If only I had arrived here sooner ...". To complete the fragment and turn it into a real sentence, state what you would have done:
"If only I had arrived here sooner I would have been able to spend more time with you."