Can I tell my girlfriend that fatherhood and motherhood are fundamentally different?
Strange. Why do you think they're different ?
because a father and a mother have different DNA.
It's ok if you think fatherhood and a motherhood are 2 different things. Just that you cannot make your gf accept that. It's just your opinion.
Am I a good dad if I tell my gf that she's a women and cannot comprehend this?
You will be a single dad if you do that. You need to accept that you cannot force your opinion on her.