GPL seems to be apart of PLL in motherboard bios voltage. other than that i can not find a definition for GPL other than licensing jibberish on google. PLL doesnt really need to be adjusted unless using extreme cooling.
found a few references to GPL
4. In the THG chart listed in Item 1 above, what is GPL ? Is that the same as CPU PLL Voltage ? (Min Setting of CPU PLL is 1.81592 v)