Would love to be able to use my GitHub username on GitLab as well. Currently occupied by a dormant user with zero activity over the last 10 months, no repos except a fork of GitLab CE and no added commits to it. Of course I don't know if he has private repos or not.
(I'm not sure why I got downvoted into oblivion. I stated my honest opinion--I haven't signed up for a lot of services where I can't use the username I want.)
GitLab.com is growing fast, but most people should be able to still claim their username.
Of course, you always have the option to run your own instance relatively easily [0].
[0]: https://about.gitlab.com/downloads/