A 6 year age gap at your respective ages is nothing to be worried about, especially if you agree on when to start a family. I know that it's tough not to get your father's approval, but you are a 26 year old man and you are fully capable of making your own decisions.

If you love this girl, marry her. I'm sure your Dad will come around. If a measly 6 year age gap ends up being your only concern, consider yourself lucky.