If you are travelling in Y but are *A Gold, then the 'rules' require that you have both your BP and Card (see extract below). Of course, this may not be uniformly adhered to by lounge attendants and IF your BP correctly states your *G status then well done - but if asked for a card and you don't have it on you, the lounge has every 'right' to deny you access.

http://webwork.staralliance.com/book/LoungeAccess.html

Star Alliance Gold Customers Travelling with Paid Economy Ticket

Star Alliance Gold customers are identified upon presentation of a valid Frequent Flyer Programme card issued by any of the 18 member carrier which shows the Star Alliance Gold logo (see Star Alliance Gold signs) plus a boarding card for any same day departing Star Alliance member carrier operated flight. These customers have access to any designated Star Alliance Gold lounge at the departing airport, regardless of the Star Alliance carrier they fly. They are entitled to bring one guest who must enter the lounge at the same time.