GPs are what Dr. Hatch calls 'human guinea pigs.' They are used as experimentation subjects and are forced to wear metal collars that administer severe electric shocks when an attempt is made to speak or escape. GPs are usually from the drugs, and are usually criminals, addicts, or alcoholics.
Hatch uses the GPs as human experiments to use as however he pleases and as prisoners. He seems to think that he is saving the used to be thugs and criminals and giving them a true purpose to live for.
Sharon Vey (possibly, without collar)
Raul (possibly, without collar)
We first see a GP after Michaels birthday dinner outside PizzaMax. He is acting in the pretense that he is robbing Michael's mother and our protagonist shocks him. Hatch leaves the GP behind as he kidnaps Michael's mother.
Later when Jack and Wade are captured after assisting Michael to the Elgen compound we see them treated as prisoners, wearing the electric shock collars. Michael is told that the collars inhibit escaping and speaking. We see Wade restrained in a chair with the collar still on. Hatch orders Michael to shock him as a show of loyalty to his cause. Michael refuses and is thrown into Cell 25.
Near the end of the book, Michael lets all the GPs free causing a prison break full of chaos. A battle starts in the compound and Hatch flees with the glows that are loyal to him
We don't see too many GPs through the book except Jack and Wade who left with Michael in the end of the last book. They have tracking devices in their arms, which Michael frees and are technically not GPs after that.
Raul from the ranch workers who are forced to help the Elgen could have been a GP but is not determined. He does seen to be like a prisoner and a servant for the Elgen who says that they treat him like a dog.
Hatch also has what may be GPs in training at the Peru facility dressed in pink jumpsuits (they are men so it's designed to bring down their self-esteem) and watch the same program over and over. The point is to drive you to the brink if insanity and submit yourself to the Elgen. If your still stubborn after the videos you are throw into a cell much like Cell 25 for a few weeks, told that you are going to be killed. Thy pull you out and say they've reconsidered if the prisoner will cooperate.