gr4vy_id
here is the name of your instance.
{gr4vy_id}-sandbox-saml
{gr4vy_id}-production-saml
urn:auth0:gr4vy:{gr4vy_id}-sandbox-saml
urn:auth0:gr4vy:{gr4vy_id}-production-saml
https://auth.gr4vy.com/login/callback?connection={gr4vy_id}-sandbox-saml&organization={gr4vy_id}
https://auth.gr4vy.com/login/callback?connection={gr4vy_id}-production-saml&organization={gr4vy_id}