Good news!!! I received the following response from GitHub Support:
I wonder if this could be a case of invalid characters being copied alongside the GPG keys.
Export GPG key and copy directly into the clipboard on the CMD using clip:
gpg --armor --export <key> | clip
Then head over to the SSH and GPG keys tab of your GitHub settings page, add new GPG key and in the input box paste the contents of the clipboard.
Let me know how you get along.