Originally Posted by JonNYC
It's official policy and to be found in numerous places including in writing in the BA Executive Club Handbook I got last year and on the website in several places.
Here it is from Ask BA in just one of a few places.
* Gold members can enjoy British Airways Lounge Access anytime, regardless of the airline flown that day. Gold members may also relax and revitalise at the Arrivals Lounges at London's Heathrow and Gatwick airports, and at Johannesburg International Airport, regardless of the class of service they are flying. Please note that Gold members cannot bring a guest into any of the Arrivals Lounges. British Airways’ Arrivals lounges are available to Gold members or passengers holding a ticket booked in Club World or FIRST.
Enjoy Worldwide Lounge Access
As a Gold member you and a guest have access to over 250 lounges worldwide. You can also take advantage of Arrivals Lounges and Lounge Access Anytime+, allowing you access to lounges even when not flying with British Airways. For full details, click here.
+Access to these lounges is for Gold members only. When flying with British Airways, Gold member can bring one guest.
I have used BA lounges when flying other non-OneWorld airlines in Europe and the US. Never had a problem (even when not flying in the old days). Lounge Access Anytime is only good for BA lounges.