Is GPP out of control? I really don't think so. GPP is too hard for most people to do. Is the term GPP being abused? I bet so!
Here's a great look and definition of what GPP is:
The point is that GPP is dependent on your goals (your sport, etc.). GPP is not one size fits all.
Now, all that said. A few point on your concerns. First, you said you did roofing for a long time ... so, you built up a good level of general fitness (GPP as it is loosely defined). I've done similar when I did landscaping for a summer. I've also had far better "GPP" when I am on the judo mat regularly.
However, many people in modern society have absolutely shitty levels of preparedness to do ANYTHING -- walking, raking leaves, etc. For these people, the fluffy definition of GPP may apply ... their "sport" is "life". And they need to up the general qualities to do "life stuff". And just doing the regular life stuff won't get you much better at it ... you'll adapt and plateu (no overload) ... and then you're stuck ... aching everytime you lift your son or daughter.
Now, for an office worker (who never does anything remotely physical), pounding away with a variety of implements (including the kitchen sink) will be beneficial -- I think. But it isn't the only thing. Getting strong is going to need to be a large percent of the training work. But remember Dave Tate's Education of a Powerlifter (or so) series. The first thing the newbie did ... was GPP work ... building up the capacity to do the work needed to be a powerlifter.
As to the difference in implements ... barbells and dumbbells have a certain center of gravity and a certain amount of stability. Fill a keg with water and its center of gravity will move around while you clean or jerk it ... different than a barbell.
Grab a sand bag. Rip it off the floor ... the slack in the bag will do ... weird ... things. Jerk it ... diffent slack. The sandbag is fairly stable ... but the gripping portion makes it unstable. The keg is pretty unstable but you can get a constant grip on it. With a barbell you have a good grip and it doesn't shift on you.
Should you do all of these every day? You know ...
Monday: Kettlebell C+P
Tuesday: Barbell C+P
Wednesday: Dumbbell C+P
Thursday: Keg C+P
Friday: Sandbag C+P
Well, maybe not ... but on second thought, I bet you'd be a damn beast if this was all you did for several weeks.
Humm, to quote a wise man, try it and see what happens.
PS Since I only have the barbell (no heavy dumbells, etc) I can't do this right away. I do use sandbags at a friends place once a week. Got a keg I need to clean out and fill with water ... darn welding. I'll get back to you when I can try this ... kettlebells are too expensive though!