My wife is coeliac too, plus has some other allergies. We've often found GFML service to be variable and the really stupid thing is that GFMLs often include shellfish, which my wife is also allergic too.
We didn't get a great service in F (both ways), LHR-JNB last year. We spoke to the CSD and one of the comments made by her was that they had been asking BA to put ingredients labels on the special meals given the importance. So might be worth mentioning that to customer relations if you contact them. If enough people raise the point they may change. Plus *maybe* they would have spotted the issue with your meal.
My wife takes some food with her, on the basis that she may not get anything suitable to eat on board, which might make sense if you're in economy but really not when in F!
Due to the issues with her food we were given 5000 Avios points plus a bottle of champagne on board. We didn't ask for any compensation as that wasn't really the issue, it was proactively provided by the cabin crew, so we weren't going to say no
