I had one of those G-techs for about 6 months. Seemed pretty accurate.
I installed a pivot speed meter X, which delimites the speedo, and also gives, 0-100/200/300/400mtr times.
I then compared the 2 running at the same time over the 1/4 mile distance, and there was just a .15 difference in the reads on 2 seperate runs. With the G-Tech giving the faster reading both times.
I then lent it to a mate who has run his car down the 1/4 with 3 runs between 15.2 and 15.3 sec. He then used the g-tech and recorded a 15 flat (not down the strip). So they would seem to be accurate to about .2sec. Not bad for the price to give you a fair indication of your times.