I don't know if they have improved over the years, or what current technology they use to measure, but my old gtech was super inaccurate. The unit I'm referring to was purchased about 7-8 years ago, and it consistently measured .5-.7 sec off. The bummer was that the inaccuracy was plus
OR minus by that much, so it was impossible to judge performance.
The unit I use now is called a Performance Box. It's made by Racelogic and it uses GPS to measure movement, speed, etc. The results I get from the P-Box are always within .1 from actual track times, so it's insanely accurate. It's not cheap, with the base P-Box (what I have) starting at $500, but it's top notch stuff for tuning and having fun. Here's a link:
Product_Overview