I don't know if you can turn the GPU on by your self.
I used '' on the turn on and turn off because the GPU isn't actually off, the system reduces its performance to use less energy, it's kind of a low consumption mode. If there's a way to tweak that manually, I don't know how to do it, I just knew that this was the way that dedicated GPUs behave on a notebook. Although this a config related to MoBo or the BIOS, so it doesn't means that all notebooks with dedicated GPU work that way. I have a friend that can play unplugged normally, and sometimes his Fps drops when playing plugged, it all depends on the system. After you try it out tell me the results please