The Star Alliance Reference Guide is clear. Only one form of proof of *G status is required ...

"Star Alliance Gold (SAG) customers travelling in any class

Star Alliance Gold customers have access to any member airline lounge that displays the Star Alliance Gold logo. Customers will need to present a boarding pass for a Star Alliance flight departing from the local airport. If the boarding pass does not carry the Star Alliance Gold indicator, the customer will need also to show a valid Star Alliance Gold card. Customers are entitled to a maximum of one guest. Restrictions apply for United MileagePlus Star Alliance Gold customers wishing to access United Clubs within the U.S. "

http://rg.starallianceemployees.com/lounge-access.html
https://portal.staralliance.com/cms/..._FINAL_web.pdf