I know for current Nuvi's there is nothing to do as a speed warning is the default behavior. It shows your current speed and the speed limit and flashes red if you are speeding. There is NO audible alert available unlike the tomtom.
Actually they do .......... well my 1690 does at least.
The audible warning is a 'bong,bong'
You get and initial 'bong' when you first get proximate to the camera. The 'bonging' will come back on and continue if you are above the Camera set speed limit'
It does look like mp3 warnings cannot be installed though (from my testing so far), perhaps because the unit doesn't have either FM or Bluetooth music support.
So it would appear like you cannot replace the 'bong' with 'Warning, GATSO Ahead' or something similar.
Sorry I can't help the original poster, I am not familiar with his unit. On the 1690 the Speed Camera warnings are part of the NuLink! subscription (1 yr free in Europe) and are enabled the moment you first switch the unit on.