Originally Posted by r2d2

There are a lot of vague policies with codeshares and some members here on the AY+ forum recommend avoiding codeshares altogether (I don't personally go that far).

However for the extra baggage allowance for Golds on codeshares, the Finnair website has quite clear wording:
You’re entitled to one extra bag (max. 23kg (50lb)) when travelling on Finnair flights (AY code on the ticket and flight operated by Finnair or Norra) and all the flights in one direction of your itinerary are operated by Finnair, regardless of travel class.