Gotti was arrested for the McBratney murder on June 3, 1974. He pleaded guilty to a reduced charge, and several months must have passed between his arrest and the start of his prison term. I believe he served two years on that rap, and Angelo Ruggiero was imprisoned with him. Gotti was still in prison when Carlo Gambino died in October, 1976, so it's possible that Tommy DiSimone was in Lewisberg with Gotti--depending on how long Tommy was in Lewisberg.
But, was Tommy really in Lewisberg at all? We have only Henry Hill's account. As I said in an earlier post, Henry Hill said Tommy asked for his bail to be revoked, but that didn't necessarily mean he'd be incarcerated in Lewisberg. And if he did go to Lewisberg, why would he hang out with Ruggiero, Gotti's best pal--and, as you noted, possibly Gotti's cellmate?