No need to be *G - if you are travelling biz, you are entitled to use any *A carrier's biz lounge (some minor exceptions). You can't invite a guest, which *A travelling even in Y can do.

See here: http://www.staralliance.com/en/trave...s/lounges.html

Lounge Access for International Business Class:

Customers who present an international Business Class boarding pass, regardless of the Star Alliance carrier they fly, have access to all members' designated business departure lounges. Guests are not permitted.