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.