I make it a rule not to spout about stuff that I know nowt about, as I have never trained a horse to GP then I can't begin to suggest that it may be possible bitless. I read recently (and Di shows a video here) that there are very few folks who ride bitless at the highest level and any that do are previously trained bitted so it's not quite the same as starting at the beginning. I have trained passage and piaffe with a couple of my horses in the past and I think the energetic, hot blooded ones could possibly have been able to be trained without a bit but I'll just never know so I'll not try to surmise.
Let's just say that I'm finding depths to my training that I hadn't thought possible and if I was twenty years younger I might have the time and energy to prove the theory; as it is I'll just rely on someone else to prove it to me.
Just short and sweet tonight but next I'll have a quick look at some of the bitless options.