While AC might invite some OA *G members to use that lounge, *G members are only able to use that lounge without a specific invitation if that lounge has a "Star Alliance Gold" sign outside the lounge. Since I have never seen this lounge, I don't know if it does, but if its a Delta lounge contracted for use by AC Business Class passengers, then I would bet it doesn't.
The rules according to Star Alliance are as follows: (Bolding is mine)
Star Alliance Gold members and a guest can access lounges around the world -- no matter which class of service they travel in! Just look for the Star Alliance Gold sign outside the lounge and present the ticket for your same-day Star Alliance flight, your boarding pass and your frequent flyer programme card featuring the Star Alliance Gold logo. Guests must enter the lounges with an eligible Star Alliance Gold member and must depart on a Star Alliance member airline, but may travel on a different flight.
As with every rule in the airline industry, personal experience will often differ from what the exact rules dictate.