Even if the numbers arent accurate, it can still be used as a measurement tool to show if you've gained horsepower.

Lets say on a dyno before and after an exhaust modification you make

Dyno : 200 whp
Dyno After : 230whp

And the Gtech said
Before: 215whp (higher tha n the dyno but could also be lower also! margin of error)
After: 235-250whp

Even though it would be inacurate on the EXACT numbers.. Before is still < after.. So you know you gained power...

Personally I dont know if its accurate vs REAL dyno pulling (which of course it cant be perfect but still. ) Or if its accurate while being used on a 1/4 mile track vs the electronic timing...

But not being accurate to the numbers dosent mean that you cant see HP gaines and losses..