There has been constant variation in the interpretation of the "rules" amongst the different Star member airlines regarding the Star Gold Extra Baggage Allowance. Some have different rules based on weight, route or class of service.
In the new United 1K Benefits Guidebook, under Star Alliance Gold Benefits it clearly states:
Priority Baggage Allowance, which allows one additional checked bag for a total of three on any Star Alliance flight in any class of service.
This seems pretty straightforward to me, leaving no doubt about any variations. Could this just be wishfull thinking or, like the recent change in common booking codes amongst member airlines, does this now indicate a common agreement on all Star carriers, for any Star Gold?