https://about.gitlab.com/product/pages/ doesn't warn that the username in <username>.github.io must be lowercase
If you want the website to be available at https://username.gitlab.io, go to your project's Settings and rename your project to username.gitlab.io. This is what we call a user page.
So, I created my repo named GarlicoinAccount.gitlab.io and followed all the other steps, then spent an hour or so fighting with the thing because it didn't work... Until I renamed the repo to lowercase garlicoinaccount.gitub.io
Maybe https://about.gitlab.com/product/pages/ can be amended to warn the username must be in lowercase (or perhaps the repo name shouldn't be case-sensitive at all?)