I'd say with a digital stopwatch. Since you have an automatic, that means you have at least 1 hand free to operate it. Just pay close attention to your speedometer and click it off when you hit 60 mph.
Unfortunately, on the dragstrip you only get 60', 1/8 mile, and 1/4 mile times.
I have no actual experience with a G-tech pro. But in my observations they tend to be slightly optimistic. With that I mean, when ever I hear somebody quote a 1/4 mile time for their car clocked by a G-tect device, it usually sounds a little faster than it should be.