I've needed to call them a few times in the last couple of weeks, and like others here have noticed a marked deterioration in response times, with 10-20 mins being the norm.
Interestingly when I called yesterday the agent asked how long I'd waited, and I said about 20 mins. I asked if it was due to short staffing or just being busier with travel picking up. She said neither, but rather the phone number for the GGL was appearing prominently in Google searches for people needing to call BA, and the team is spending an inordinate amount of time answering calls from non GGL members, and redirecting them to the right place.
Worth saying though that when you do get through, the agents are as high calibre and professional as always.