Yes diarrhea is a normal reaction because the juice is very cleansing, you could reduce make the juice a bit less green and use more carrot or Celery if it's too bad but it can be a bit rough for a few days, The main thing is to stop the bleeding initially and two days won't do it, but it will stop if you stick with it.
I never did the GAPs diet or SCD diet but something similar that I found suited me. It's just a version of the SCD diet. I don't have a problem using carbohydrates like potatoes and rice, mung dal and even some wheat and I got into remission using those things so I don't entirely agree with it but some people may benefit from excluding things like that to begin with. Other things like fermented foods are good but I think they are wrong about vegetables, I think fresh green juice is much better than meat and fish stock for healing but the main thing is that it eliminates all junk food and especially Sugar and bad fats
I believe once you get the minerals you need from green vegetables etc, optimize your vitamin D and normalize you bowel flora with probiotics if necessary and avoid all refined foods until you are well healed, you can then eat almost anything in moderation as long as you stick to a largely healthy wholefood diet and organic if possible.