What exactly is it? I have an idea of what I think it is but if anyone knows a little more about it that would be great.
GPP stands for (as you may know) general physical preparedness. It is roughly equivalent to work capacity which is roughly equivalent to conditioning. If you see any of those terms they are more or less talking about the same thing.
If I am wrong someone will have to correct this next part, but I believe all of that can be equated to a combination of strength endurance more or less, and the general ability for your nervous system to handle the stress of continuous exertion.
Usually space is set aside in a warm-up, in a second workout during the day, or on off days to work GPP by way of extended anaerobic activities. Sled-dragging, farmers walks, strong man training, various kinds of carrys, etc are all good examples.
I see it more as recoverability. Things like sled-dragging, sledgehammering and tumbling drills get the blood flowing without really causing any muscular or nervous stress. As a result, workouts like these should increase your ability to recover and supercompensate from your real training.
I say should, because my experience is limited, and I am loath to give credit where it is not due.
I think the term "GPP" has been twisted a bit, so the definition sort of depends on who you ask, kinda like "overtraining", "toned", or "real breast."
Dave Tate has said that anything not directly a part of your sport can be considered GPP. For powerlifters, this would be everything except the squat, bench, and deadlift. So, reverse hypers, sled dragging, tricep extensions, and elliptical machines all fall under that category.
Jim Wendler has mentioned that, according to old school Russian training textbooks, things like long walks in nature and playing soccer were good GPP activities.
Chad Waterbury had a great article a long time ago, GPP ASAP, I don't know if his thoughts have changed on the matter:
As far as I'm concerned, I try to only use the term "GPP" when referring to activities performed by people who either compete or participate in recreational sports, regarding training sessions designed for active recovery (not specifically fat loss/strength/endurance/speed, etc.).
For the Average Joe who works 40-hours a week, hits the gym for an hour, four days a week, but doesn't otherwise get off the couch... I'd call it "cardio", "energy systems work", or something else. But that's just nitpicking over the phrase, I guess.