The Bell curve of cylists - how fast are the average TR users? This thread might be of interest. There’s a bell curve on there : The Bell curve of cylists - how fast are the average TR users? - #6 by unstoppable
(Power distribution of TrainerRoad users : Staminist Cycling Forum)
edit: Bare in mind it probably doesn’t tell the whole story as it’s a sample of cyclists using a training platform called “the sufferfest” so I would guess the average would be slight above the actual average of cyclists. Although that’s probably the same for these big gravel rides you’re talking about.