Trading for Jon Gray doesn't make sense IMO--it is a sideways move. Your top 3 starters matter most in the postseason (will pitch twice in a 7 game series) and the Red Sox have Eovaldi and Sale as 1-2. That leaves E-Rod, Pivetta, and Houck under consideration for the third rotation spot. But is Jon Gray clearly better than E-Rod, Pivetta, and Houck? If not, it doesn't make sense to trade for him.
Kyle Gibson is having a better season than Gray. If you believe that Kyle Gibson is for real, it would make sense to trade for him as he would represent an upgrade over E-Rod, Houck, and Pivetta. I just don't know if Kyle Gibson is for real.
Gray has always been a bit of an enigma. He has one above average pitch (his slider) but he has never been able to complement his slider with his 4-seamer and that has kept him from blossoming into a top guy. His velocity has declined (slightly) over the last couple of seasons. Maybe if the Rockies were better at developing pitchers, Gray could have been a top starter. I suppose there could be some untapped potential there, but his 4-seamer has always been a problem for him and his changeup is nothing great and he doesn't throw it that often.
Jon Gray is capable of having a really good postseason start, but E-Rod, Houck, and Pivetta are capable of that as well.
While Gray would likely be a sideways move, acquiring a starting 1b would be an immediate upgrade, since the Red Sox have the worst or nearly the worst OPS out of the 1b position. In addition, I read this morning that the Red Sox's top relievers are on track to pitch more innings than they ever pitched before. Assuming that is true, there is a real concern about their continued effectiveness. Moreover, while acquiring an upgrade to the starting rotation is almost impossible to do, there are a plethora of relievers on the trade market that would be an upgrade over the Rios-Workman group.