We call this solution, of authenticating at an external IDP, inbound identity. Gluu leverages its passport.js component to achieve this solution.
If you want to send users to an existing SAML IDP, follow [this document](https://gluu.org/docs/ce/authn-guide/inbound-saml-passport/).
If you want to send users to an existing OAuth AS, follow [this document](https://gluu.org/docs/ce/authn-guide/passport/).
Feel free to open a new ticket with specific questions if you run into any issues. Also search here on the forums, there should be quite a few inbound identity tickets available for your education as needed.