Does anyone know how to disable gps that has been installed in a company truck?

Is it your company? What's the situation and why do you want or need to disable it?

Yes I do

But a gps in a company vechile is to track how you use it, what speeds you are going and also where the truck is.

If you wish to have it turned off, merely ask your company to disconnect it.

Your company will know if it is disconnected, since milage on the truck and their tracking milage will be different. And of course if they don't give you permission they would merely fire you and deduct the cost of fixing it from your last pay check.