[OmniAuth] Autologin with HTTP header

I have a ProxySSO who is reponsible for authentification and transfers the authenticated users to backend GitLab by $REMOTE_USER (username) in HTTP header. GitLab side, I just need to receive the variable to have autologin feature, anyone knows how can I do it ? It seems the offcial doc is not very clear : https://docs.gitlab.com/ce/integration/omniauth.html

Any suggestion will be appreciated !