You’ve got a substantially larger extension there for the Gates GT3 I believe, but that’s not suprising given the somewhat improvised nature of my “test equipment”.
What is interesting is that you have a much smaller difference between your Gates GT3 and the regular Fiberglass belt, so either my OEM Fiberglass belts were particularly bungee cord like or my GT3 sample is less stretchy.
Whilst digging in the Gates belt data I found their spec for the fiberglass cord in their GT3 belts giving (for the 9mm);
breaking strength min 190lbs (no, stop, you’ll break the stepper motor)
tension for 0.1% elongation 5.7 lbs
Taking The_real_janderson’s measurements of a sample belt; 0.68% for 38lbs this equates to 5.59lbs tension for 0.1% elongation, which is pretty close to the spec (no tolerance given in the spec). Always nice to see consistent data points.