OP, I realise how ignorant my post above sounded and I do apologise. As I said, I have no experience of gastrostomy in small children but from what you say your daughter must be a little older than mine so I can imagine the sense of lost opportunity for her, the sheer bloody unfairness of it all (and yes, I fully understand how lucky I am to only imagine it).
My father underwent dialysis. First he tried peritoneal dialysis, which entailed having a catheter placed in his abdomen which he would then hook into a pump for the dialysis fluids to be pumped into his peritoneal cavity and then taken out. Even with him being a fully functioning adult who could take care of himself, be mindful of his hand hygiene and who took very good care of his catheter and the surrounding area, as this is so prone to infections, it was extremely worrying and time-consuming. My imagination fails when I try to picture a small child undergoing something even remotely similar, even though that will probably be for the best, just at it was for my father. You cannot let your daughter slowly starve or constantly gag and vomit and lose weight as a result, just as my father could not slowly poison himself with the toxins that his kidneys could no longer clear from his blood, but that doesn't mean that it is easy to accept.
I hope that you find a way to come to terms with it all. I remember my father resisting the procedure and trying to put it off, even when feeling like shit from the toxins and the excess water that he couldn't get out of his system anymore. Our desire, our need to protect our children is so strong that this must hurt a hundred times worse than anything that could happen to you (and you must wish that you could take it onto yourself, to at least share the burden so she could be better, but you can't).
I have no practical help to offer, but just remember that your feelings are valid and understandable and that this is so hard that you shouldn't have to do it alone. If you have support, take it. If you need help, ask for it. It's not just for you, you need to stay strong and sane for your girl, so if you won't do it for yourself, then do it for her.