He was never an "official" acting capo so to speak. He did operate sometimes in an acting capo capacity when he was dealing with certain business and people but that's because his capo (Toddo Aurello) allowed him to do so and used him in that capacity, or when Castellano wanted to deal directly with him on some constructions and unions matters.

But as far as I know, the soldiers in the crew didn't kick up to him or didn't ask for his permission before doing certain business.

It's common for made guys to sometimes act as acting capo for a specific matters but, to me, it's different from when a capo goes to jail and the family appoints someone to run the crew in his absence, as a sort of "official" acting capo. Gravano was never in that position.

He was a soldier (and only a soldier) from 77 to 86 and he became a full capo in 1986 after Gotti took over.