Luffy does not require power to destroy or through the gates of justice. it proved luffy can get inside in and out impel down and straight to the marineford without destroying that shit. Of course the fact that he could do it with Haki.
I don't remember the gate being made of seastone. And even if they were it doesn't really matter other than the fact that its harder than regular materials. Luffy would have his usual speed power all the way up to the moment he touches it and then its only for a split second and I think he can counter that amount of time with his haki.