Well, if you aren't talking about this as a costcutting measure, the reason you would trade Gio is it is clear you will not re-sign him. He has been the rare sox to enter FA, and the other was Rodon, and Rodon is gone (after the detour).
So you could also just say "we are going to trade Giolito away because we won't re-sign him, get value back, and sign a different pitcher we can get for the next 3-4 years".
If you are just completely capped, you probably aren't trading Giolito it's true.