Update (2017-12-28)

After adding a deployment key to a repo, I had to manually go to Admin panel, then running Rewrite '.ssh/authorized_keys' file. Then I was successfully authenticated again.

Original post

I created a new gogs user for my buildbot but I was unable to clone the user's own repos even though I had added the SSH key of my current UNIX user.

$ git clone ssh://git@git.example.com/buildbot/foobar.git
Cloning into 'foobar'...
Gogs: Key access denied
fatal: Could not read from remote repository.

Please make sure you have the correct access rights

I think one of my other gogs users was already registered with the same SSH key without my knowing. So it worked correctly after I created a new SSH key pair. (Be aware that the below command deletes a user's SSH keys and may be disastrous.)

$ sudo -u buildbot -i bash -l
buildbot$ rm ~/.ssh/id_rsa ~/.ssh/id_rsa.pub
buildbot$ ssh-keygen -b 4069 -t

Then the Gogs profile with the new public key

buildbot$ cat ~/.ssh/id_rsa.pub

And it all worked as expected again

$ git clone ssh://git@git.example.com/buildbot/foobar.git
Cloning into 'foobar'...
The authenticity of host '[git.example.com] ([ba:be:ca:fe::1337])' can't be established.
RSA key fingerprint is SHA256:7xm2gVtHc41V0nh8NussuA7lNAzQwh5f9yn+5Mcpdmg.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '[git.example.com],[ba:be:ca:fe::1337]' (RSA) to the list of known hosts.
warning: You appear to have cloned an empty repository.
Checking connectivity... done.

... I just have to confirm before posting a bug.