FWIW, I totally get it and I have also lost a child. I lost my only girl after four boys.
Before I had her, I desperately wanted a little girl. Got pregnant for what I knew would have to be the last time, and immediately got into "It'll be a boy" mindset. I imagined I would have five boys and resigned myself to it.
Then she was a girl, very small , very premature but as soon as we could reasonably feel she was going to be fine, we relaxed and for seven weeks, I felt I was living in a beautiful pink haze. Then we took her home and she died unexpectedly of SIDS. Devastated would be putting it mildly.
I would have been just as devastated had one of my boys died. And I know that a lot of my neighbours did think that it was especially devastating because she was a girl. But a friend of mine had her fifth boy at around the same time and I do envy her because she still has her baby. But I do have slightly more difficulty seeing a baby girl who is the age that she should be, than I do seeing a similarly aged baby boy. Because my boys were that age once and they lived so I don't feel that same sense of loss. I am grateful to have my boys, without them, I don't know how I'd get up in the morning.
I do feel I suppose that I "get" gender disappointment but that it's a feeling from my "other life" when I hadn't lost a child and I used to be normal and worry about normal things. And I think gender disappointment is a fairly normal thing. I didn't like to call it that, though. I never felt disappointment in any of my sons, they are all fabulous. The disappointment I felt was in the non-appearance of the daughter, rather than in the appearance of the son, IYKWIM. I have seen it called "gender desire" which is, I think a better name. And one of the reasons I think it's normal and common is that people who have "one of each" are statistically far less likely to have a third than people who have two the same.
FWIW expat immediately after dd died, I read a lot of threads on MN which seemed to me to be trivial. A lot of my thoughts were along the lines of : "Having difficulty breastfeeding, having to give formula? At least you have a baby to give formula to, you ungrateful cow!" or "Your dd is being cheeky? You should be bloody grateful to have a dd!" but I stopped myself from posting.
When you compare any problem to the loss of a child, it's going to seem trivial. And luckily for most people, those problems are going to be as bad as it gets.
And for ages, I only posted in Bereavement because bereaved Mums were the only ones I could relate to.