Not sure about GP, but that’s basically what we did under “SAFe” (Scaled Agile Framework). PI planning means taking most of a sprint to plan everything for the next quarter or so. It’s like a whole week of ticket refinement meetings. Or perhaps 3 days, but when you’ve had 3 days of ticket refinement meetings, it might as well be the whole work week for as much a stuff as you’re going to get done otherwise.
It’s as horrible as you’re thinking, and after a lot of agitating, we stopped doing that shit.