This is true, but that is not convenient at all.
In the specific situation I described above, I was in a hurry to get the bathroom repaired so my kids could use the shower again. I have a trailer and could have unfolded that, but that would have added an hour of extra, unproductive time. Same for renting a truck. I just wanted to get one sheet of drywall, come home, cut it and put it up.
For bigger jobs, like cutting bushes and trimming trees, I would definitely get out the trailer. Last year I stacked 80 bags of mulch into it. I would not need the Gravity to hold 80 bags of mulch, nor would I want to try to put 80 bags of mulch into it. It would create a huge mess inside which would be hard to clean up.
That is possible, but putting a 4x8 sheet of drywall on the roof is asking for trouble. It would need to be tied town really, really well. And air could still get under it, lift it and break it. Drywall isn't very strong.
The roof would work well for a few long pieces of smaller wood, like 2x4s. But think of the range hit!