I believe the OEM/Gates timing belts are indeed Gates PowerGrip GT Belts
http://file.lasersaur.com/docs-thirdparty/The_World_of_Timing_Belts.pdfThe PowerGrip GT Belt Drive System is an advance in product design over the Gates' older,standard HTD system. The PowerGrip GT System, featuring a modified curvilinear belt tooth profile, provides timing and indexing accuracy superior to the conventional PowerGrip Trapezoidal Belt System. Plus, PowerGrip GT Belts have a higher capacity and longer belt life than trapezoidal belts.
T-39SECTION 9 DESIGN AND INSTALLATION SUGGESTIONS
8. Belts are, in general, rated to yield a minimum of 3000 hours of useful life if all instructions are properly followed.
https://itstillruns.com/convert-hours-engine-miles-5978869.htmlUse the number you come up with to estimate how many miles are on your engine. For example, 1,235 hours on an engine equates to approximately 74,100 miles.
Multiply the amount of hours on your engine by 60.
3000 hrs = 180,000 miles minimum useful life.