Hey @Dave_Feldman thanks for asking! There's an update on the status of Google SSO for custom domains over here http://community.retool.com/t/sso-for-custom-domain/23159 - let me know if that's the info you were looking to confirm 
tl;dr we're looking into it on the Retool side, but it will take a bit more work on our side to support SSO for custom domains.