So this has kind of caught me a few times and thought I might stick it here as a future note to self.
When attempting to push to a Gist using SSH authentication, the remote host needs to be in the format that support SSH.
e.g; [email protected]:xxxxxxxxx.git
Command to check:
git remote v
If the remote host is a different format:
git remote remove {remote_name}
git remote add {remote_name} [email protected]:xxxxxxxxxx.git
THEN you should be able to push your Gist.