Gryphon absolutely has enough space to be used with both hands.
And Mihawk referred to Shanks as an one-armed has been. So he referred to Shanks being unable to use both arms in combat and not just "losing his strong arm". Would be pitiful enough for Mihawk to have a rival that uses his blade only with one hand.
So let's assume Roger was indeed a swordsman by definition. The WSS of his time. Was he > Mihawk? That would make Zoro's dream less relevant, because he will not surpass Roger and
only claim the WSS title of the current era, while Luffy will surpass the old era. Obviously, Luffy was made to be the Roger of the current era. There is not the slightest parallel between Zoro and Roger.
Where is Roger's black blade? Roger, with an inferior blade, CoA and one handed > Mihawk, how?