This is the issue — Old Gaban is logically the weakest or one of the weakest top-tiers. A good point of reference would've been to have Sommers clash with him again for example, then we would've seen a before and after.
Sommers was more than likely a bum to Zoro before, and now he's not, so clearly Sommers is far above any high tier but how does he compare to say Yamato, Law, Gaban, Ryokugyu etc... Obviously Oda doesn't care about such things but without a point of reference, we'll never be able to scale these guys beyond "Super Sommers is significantly stronger than King".