The idea of renaming an account on Github concerns me because an account's repos may be referenced by others' git submodules. I'd hate to find one of my repos inadvertently referencing a repo I did not intend to reference.
In cases where an account does have repositories (user initiated renames) the old URLs/repos will redirect as long as another user doesn't take the old name and create repos with the same name[1].
[1] https://help.github.com/articles/what-happens-when-i-change-...
For both Gemfiles and submodules this is an extra reason to fork the original and reference your fork rather than directly link to the original (unless it is big and reliable and unlikely to to be replaced e.g. rails).