Star's published rules are clear that this is fine - only one form of status proof (gold card OR boarding pass showing *G status) is required for lounge access. What is less certain is whether lounge dragons, checkin staff etc know the Star rules they are supposed to be following. If I were you I would be asking at checkin where the lounge is and whether you need an invite to the lounge - if checkin gives you a printed invite you are less likely to be knocked back than presenting just your gold card to lounge staff who think they make the rules. If by chance the checkin staff says you don't have access you at least have a chance to speak to a checkin supervisor.

I believe BR lounges have been known to turn away *G flying with other Star airlines in some ports, which is against Star rules (although 'space' considerations are open to interpretation). But if you are actually flying BR (and in fact crediting to them) you are likely to be OK.

And for the record I have been upgraded from Y to PE by BR in Asia while holding *G with another airline (and crediting to that airline). I had no trouble with lounge access, but I had both gold card and *G on BP.