I changed my username from geoff-codes to geoff-nixon.
So, if you're looking repos or gists that began with:
https://github.com/geoff-codes/...,
or
https://gist.github.com/geoff-codes/...
you can find them at:
github.com/geoff-nixon/, or
gist.github.com/geoff-nixon/.
Unfortunately, GitHub doesn't allow forwarding/creating gists from an old username.