We are currently using FusionAuth (
https://fusionauth.io/ ) with OIDC and it works great with GAS/Apache.
Any OIDP/SAML IDP should work using the SPs shipped with the FGL instalation under "web_utilities".
We also used Genero Identity Provider for some time but it is a bit short on functionality compared to full fledged IDPs(No Password rules configuration for example).