fatal: failed to install gitlab-runner: service gitlab-runner already exists
Rename the init file with
sudo mv /etc/systemd/system/gitlab-runner.service /etc/systemd/system/gitlab-runner.service.bak
and execute again
sudo gitlab-runner install --user=my-user --working-directory=/home/my-user
cleanup the backup file
sudo rm /etc/systemd/system/gitlab-runner.service.bak