I think that everyone has their own path to the GP, but it is an entirely different experience if you train it yourself vs. buying it trained or paying someone else to train it (and I know plenty of local trainers who had someone ELSE train their horse, but claim the training themselves because um, well they showed the horse).
My path is as follows: When I was a child, I was a prodigy hunter/jumper rider and had a mentor who was so classical old school that we had rules: one day course work, one day gymnastics (usually bounce chutes on the longe, no stirrups or reins), three days on the track (we had a race track on the property) trotting for an hour to build strength and endurance, and one day of dressage (figures, lengthen and collect, etc.). Life interrupted my life with horses and I had a (with the off ride here or there) 18 year hiatus. When I got back into horses, I decided to try dressage, at age 40. SUCH A DIFFERENT EXPERIENCE! The dressage trainers I encountered all wanted me to buy a trained horse and keep it in full training to keep it tuned. I did not meet any like my mentor in h/j, who wanted to teach me to train a horse. So I bought a GP schoolmaster and went into full training. Actually what happened is that the trainer got to ride my GP horse while I took longe lessons with a British certified instructor three times a week…and very soon I wised up and took my horse and left. I bought a foal the same year that I bought the GP horse. I showed the GP horse up to PSG (not very well, but largely self training with clinics as my horse was only intermittently sound due to turnout accidents), and brought my foal from Europe. I trained her from about 90-120 days under saddle (not sure–paid for training, but she was definitely totally feral when she was imported at age 5).
I hired a natural horsemanship expert to help me with my young horse because she was so feral, especially on the ground, and I started sponsoring clinics with a former Olympian on a regular basis. I couldn’t show my mare until she was 7 because she was too feral before that. But at 7, we did training and first level, at 8 second and third level, at 9 third and fourth level, at 10 fourth and PSG, at 11 PSG and I1, at 12 I1 and I1 Freestyle (this was the year that we struggled with teaching passage), at 13 I2 (and had some medical problems this year) at 14, one show at GP (not very good) at 15, GP and got my first gold medal score but was struggling with consistent high 50s due to mistakes. We won regional and national awards at every level. At regional championships in 2015, my horse and I had a terrible injury (both of us fracturing bones) and had surgeries and about 2 years off. In 2017, I earned my gold medal with this horse, but we both were still healing. In 2018, we were still not on full form and I struggled with determining whether it was fair to continue. I decided to retire her from showing because I met my goals with her and I didn’t think it was fair to keep pushing the GP. I had a freestyle made and that would have been fun, but we had scores only to 62.6, so I couldn’t do a freestyle this year. I bought some foals. I am currently, deconstructing all of the movements to learn how to train them better (my horse is still sound) while I raise foals and I am doing a lot of early foal handling. I will do this all over again multiple times. I enjoy the training way more than showing or lessons for showing. I don’t get any satisfaction out of riding a trained horse or letting someone else train and/or show my horse. I delight in the horse learning new things and I like have the buttons put on the way that is familiar to me. I recently bought my own farm, so I can totally do my own thing. I did full training the year I was making the leap from I2 to GP and absolutely hated it so I will never ever do that again. The rest of the time, I trained my horse on 2-4 lessons per month from various local trainers and about 4 clinics a year.
It is really hard, where I live, for an amateur to find top level clinic opportunities, so I believe that I will go pro when I start my oldest young horse under saddle, even though I am doing all this with a high level, full-time corporate job.
I do think almost anyone can get to grand prix–it is not magic–it just depends on what you are willing to invest to get there. For myself, I made a lot of sacrifice of time and I went to the barn almost every single day. I did lose a lot doing that.
The ideal dressage horse can push AND carry AND is very very submissive. Those horses are rare and usually very expensive. Most of us are dealing with a horse that can push and not carry or who can carry and not push and have varying degrees of submission (my horse has a tendency to take over at shows and will open her mouth and lean if she feels any tension from the rider (shows). But each horse will teach you very important things.