And how do you prevent the thief from just removing it, without blocking the range of the gps and gsm antennas?

a fair question, the ideea was to put it inside the bar.Was intended for fiber carbon bicycles that are more expensive but i assumed  that there you do not have the problem of shielding.Lack of money and time prevent us to have it tested in real life (we have it functional at  lab model level).Another ideea was to build it in a pipe and also build the pipe from a non metal material but there is the volume problem- not enough space for battery.But that is too much for us. Anyhow, project is dead now, maybe will come to life if someone will be interested, i've spent enough money on it but i have a small toy that can send packages of data via GSM...I've placed it only for 1 inch contests

interesting, how do you power it? 

battery under the device, everithing on a cilinder of battery diametter- that was the ideea