Assuming Gray produces 1 win for the rest of the year at a cost of $1M and change, times the 2x contender's premium, he likely has almost $20M in surplus value this year. He is controlled via arb for 2 more seasons that will cost about $15M. If he produces 6 WAR over those 2 seasons, that's another $45M in surplus value.
The Astros have the following valuable prospects according to BA's midseason list (with estimated surplus value):
8. Bregman ($71.6M)
29. Martes ($33.9M)
32. Musgrove ($32.6M)
35. Tucker ($41.8M)
47 Paulino ($25.0M)
94 Fisher ($20.2M)
We can piece those values together to get $65M. We know Bregman is untouchable, and Tucker is likely off limits (they would have Q or Sale if they were willing to trade him). Musgrove and Martes are already in the MLB rotation, so maybe they are off limits too?. A package of Paulino plus Fisher is likely not enough to get Gray. Maybe Musgrove/Martes plus Fisher plus 2 other notable parts? Maybe include Whitley as a way to not include Musgrove/Martes?
I'm guessing one of Martes/Musgrove plus Fisher plus 2 other parts for Gray.