Sitting the trot is the easiest part of 2nd IMO LOL
If you go to a CDI and watch the rides, those three things are where you most see something lacking at the GP. The pirouettes are introduced at 4th level, the collection is pretty established by 4th/PSG. It is the ultimate collected moves, the transitions between those moves, and the "new gait" of the 1s that keeps so many talented horses out of the GP.
I think the big "barriers" are 2nd level, PSG if the horse struggles with changes, then I-2 and GP.
My hunter could do 2's but I never asked for 1s and he was pretty far from being a GP horse--but he could do changes and stay light, correct, straight and balanced even in sequence--I suspect if he'd had more dressage centered training he might have been able to produce a line of ones--but maybe not GP quality :)
My current guy is not at a level where we actually need Changes yet but I did play with them a bit earlier this season just to be sure we had something to work with later as I know this can be a bit of a deal breaker for some. Occasionally I play with them--but they are not our focus at this time.
In some ways I think we waited too long with our other guy to put the changes on him and was just confirmed at Third Level earlier this year (3-3).