It sounds like your dog has colitis for reasons other than diet. Sometimes the cause cannot be pinpointed. I had a dog like that and the colitis went on for some years. I had to handle it with medication and she was on Metronidazole, Buscopan, Prednisolone (steroid) at times as well as other drugs that I can't recollect right now. Has your vet suggested long term medication? It might be worth considering that - from what you have said, it sounds as if he is struggling somewhat so at least it might make him more comfortable.
I think it would be useful to have a conversation with him or her to see what the best way forward is. Have you had a stool sample analysed? Just wondering about infection, particularly Giardia. Alternatively, if you still feel that it is diet related then you could ask your vet to help you with a proper controlled exclusion diet. This takes some time and you need to start with one of the special products designed for this purpose. The snag is that dogs sometimes don't take to the food.
Regarding the Gentle, it is a shame that it hasn't worked as well as you had hoped. Presumably you have weighed the product and not overfed? Sorry to ask but from reading their Facebook page, this seems to be a pretty common scenario that can cause a dog to have frequent, loose stools.