I haven't seen this posted in this thread so if I've missed it and it's in there then I apologize.

The list of GGL benefits has been updated on the BA website and now includes access to the Chelsea Lounge without restriction provided you're flying on AA or BA. Here's an extract. It also includes the guesting policy which seems reasonable.

You can also take advantage of our exclusive Chelsea lounge at New York JFK Terminal 8 when you fly with British Airways or American Airlines. Likewise, a guest may join you when you’re flying with either airline, or two guests when flying in First. (Passengers flying with other oneworld airlines are unable to access this lounge, but can enjoy our other two lounges).

https://www.britishairways.com/en-us...-list-benefits

This resolves the question of GGLs accessing the Chelsea when flying AA domestically which is excellent news.