Years ago, theory was (plausible or not) the GPK was recommended for antenna installations more than a wavelength above terrain; and unnecessary for installations a wavelength or less in height. The advertised intention was the GPK would lower the angle of radiation.
From a practical standpoint, I'm lining up with those who contend the GPK makes no difference in antenna performance. I had an Anttron99 at about 70 feet or so above terrain, lost one of the radials in a windstorm, and it made absolutely no difference in performance.
For the belt and suspenders type of person, use a GPK because I can testify that it doesn't hurt performance.