I'm having the same issue as JohnMillimetron, I have some super low quality textures. I have deleted settings.cfg and the option for GPP_High shows up but it doesn't change the quality of the planets. After a restart GPP_high goes away and it just says "New Text" on the texture slider and going up the highest it goes to is high. After going back into the main menu screen and back into settings it says "New Text" again. No slider adjustments make any quality changes as far as I can see. (I'm on 10x rescale if that makes a difference.)
Thanks for your awesome mod, hope you can help out