These guide looks amazing:
First, I would try to consider what kind of protocol I can use to connect my WHM with Gluu Server. Will it be SAML? Will it be OpenID connect.
If it's OpenID Connect, above doc is good.
I saw some SAML / Shibboleth related doc as well in internet which can be used to create SSO bridge with WHM / cPanel server.
I believe these are good start!