As recently mentioned in JENKINS-55525 ideally it should be solved at IdP side, but in practice it is currently impossible to get user display name as a single attribute from G Suite SAML. You only get first/last name separately.
My proposal is to process configuration value as whitespace separated list of attribute names, so for existing setups there will be no visible changes but it will become possible to configure it as "firstName lastName" for G Suite users.