I don't have gps on either of my cams - it's not difficult or complicated to calculate your speed, simply take a fixed point on the vid & find it on google earth - there is a ruler on there to help measure. time taken to cover a measured distance gives speed. In this case, it will be an approximation but as near as makies no real odds. Also, most of us know where we are when an accident or an event happens, a quick look on google streetview will confirm this.
I had gps, google maps overlay and g-sensor data on my old t-eye. In less than a few minutes i was bored with looking at it.
Last year whilst walking my dog along a nearby canal, I had set my phone to log my journey. When I got home and checked my stats, the gps had me doing 50mph in a westerly direction starting some 5 miles off the coast of Wales!
Not bad, only a few hundred miles out!
I've also noticed some odd fluctuations when I'm driving my bus along the motorway. I'm maxed out at 62mph (100kph) and, generally, this is reflected in the speedo on my gps. However, there are times when I can reach speeds of up to 80mph (I don't feel like I'm going any faster, my speedo doesn't show this - but if that was on a video it could get me the sack!