When using a custom domain, Github requires a CNAME file. I think this can be done through correct merging of old and new gh-pages branches, but if I understand correctly, ember github-pages:commit will destroy the branch and recreate it, removing any CNAME file that's committed to the branch. Use git push origin gh-pages:gh-pages -f with caution for this reason. Not sure if this is an issue with this add-on, but it's worth noting.