How does one do a derate TO in this aircraft?

It would be done through flex selections, but I don't think it's modeled yet. You would have to do it manually by setting a lower throttle position. But, psst, here's my opinion. That engine model is designed to churn out 4500 to 7000 pounds of thrust. So in essence, it's already derated. Take the BR710 that I fly with.  The G550 has a rating of 15385 and the same engine on the GV is 14750. The engine itself is up to 21430. So, a LOT of operators don't derate. We accomplish max power takeoffs every time because it's thrust is cut back. Just an opinion. When I flew heavies, we derated as much as possible.

Thanks Rick