I think you will find that this device (GT200) and it's brethren (AED-651) have been mentioned on this blog several times.
Most recently the AED-651 has been mentioned here and on the Cambridge Labs web site (lightbluetouchpaper.org).
Dr. Markus Kuhn who did a bit of an investigation for the BBC television Newsnight program.
And I belive from other news items that the UK company director(s) will be providing their side of the story in court at some point in the very near future (as far as legal bretherin are concerned).
Also some bloke called James Randi is offering up 1mUSD to anybody who cares to prove these devices and dowsing in the more general sense works.
Have a google for [dowsing "bomb detector"].
As for dowsing it's self it's a fun packed area of entertainment, full of strange charecters making claims about mystical lay lines, morphic resonance, standing stones, pigeon like abilities to detect magnetic fields and mystical water channels through solid rock etc etc.
And without a doubt there are people in places of authority who belive in it enough to spend a lot of tax payers money on it.
And where there is untestable belief and money naredowells are sure to be found, selling fabled fine clothes to Emperor's ( http://en.wikipedia.org/wiki/... ).
So far the best "theory" on the "pro" side (because it sounds credible and is virtualy untestable) is that, dowsing is a way of showing a very weak "gut reaction" from the residual monkey brain.
That is it is the body expressing a physical reaction to a primitave survival instinct we have to spot subtal natural indicators colour and type of vegitation physical land features such as slight depressions etc etc. That we have learnt by experiance indicate where water or food etc might be under the ground. Thus it's a bit like "hinky" detection experianced Police officers etc have.
There is some evidence that dowsers in their "home ground" favour areas that aerial photography shows has slight distinquishing features.
As for the anti side of the argument well lets just say it's a bit colourfull and definatly adds to the entertainment factor (James Randi is a showman to put it mildly ;)
Personaly I think it makes an interesting entertainment or side show like many other supposed "mystical properties of the mind, without rational explaination" activities.
Oddly though there have been one or two "scientific trials" which show it does not work, but... by more than you would expect on a random trial...