My Existing Web System is Using the Google Sign in. I want to add that into my Arcgis portal as well. Using separate username and password for portal and web system is inconvenient for users. My ArcGIS enterprise 10.6.1 is hosted in AWS, ubuntu 16.04. It was setup by arcgis cloud builder.
If Google Sign In is a SAML/ADFS provider, you could try to set up Portal to use it:
Google isn't listed, but it could work, no guarantees though.