Every character in one piece have only one main fighting style. They can use other abilities like awakening so their fight style can change based on that but even then not by much.
King is one of few who has two styles - fighting with sword in CQC and fighting in range when using hybrid and full transformations.
Kaido has two also in that regard he fights in dragon mode and base/hybrid.
So GB at best may have two. Using his DF and then using sword.
I think GB might use his sword in combination with his DF power. Going by the fact that it has no handle I don't really think he fights similar to conventional swordsmen.
I can see him using it as an undestructable head of a root whip or something, where he swings or slings it around with his veins/roots/branches whatever you wanna call it.
It would also fit his basic fighting style and would just power it up, even if takes out the nutrient draining hax.
And I nonetheless think he will have a second fighting style. His DF is superb in creating gigantic fists. He already did so when casting his hateful forest. Those tree fists are gigantic and if he has peak CoA, I can see him using big boy power mochi like attacks.