My 2000 GMC pick-up stops running when I drive a steady 15 or 20 miles in one trip and won't start again until it cools down some, a few hours later. It has a V-6 for an engine, what is wrong with it? some have said it was the fuel pump, but I disagree, and another said it was probably the sensor, which sounds more right than anything else, but I would like a pro to tell me.

With the complexities of modern vehicles I would only be guessing without more information. The first thing you need to do is check to see if there are any diagnostic trouble codes. You can have this checked for free at Autozone or you can do it yourself. To do so you can use the ignition key; without starting the engine turn the ignition on, off, on, off, on, off, on, within 30 seconds. After a short delay the codes should be displayed. If you have a digital odometer they will be displayed there otherwise they will be displayed by the check engine light flashing. The first code will be 12 (engine not running) and the last code will be 55 (codes complete). If you have to use the check engine light the codes will be flashed one digit each three times for each digit, 12 will be flash, flash, flash, delay, flash flash, flash flash, flash flash, delay then the next number until they are complete after code 55.
If no codes show up then I would have to suspect a defective ignition module before any other problems. To check this when it won't start check for spark. You'll need a spark tester ($3-$15) to avoid possibly damaging the electronics.