We have a few of those LED
We have a few of those LED signs that flash up “Slow Down 30mph” (or whatever the speed limit is) if you are exceeding that speed, in our area that I have triggered a few times. So assuming they are using the same speed detection technology it should be possible to set a Gatso speed camera off.
I think the main problem is that only a small proportion of the Gatso cameras are actually live at any one time, they move the camera apparatus that is contained within the box to other locations as and when they see fit.
You may have been going fast enough to trigger it, but without the camera being live you would never know it.
Aim for the speed warning signs, they are always active and much more of a guarantee of fun.