Thanks for all your replies!
This morning she is not doing it as much - maybe the odd cough but not the gagging part.
I have heard that coughing may be a sign of heart troubles as well - that is why I worry sometimes. She certainly doesn't cough all the time but I would say I notice it every week - it seems worse especially when she does the gagging thing. Sometimes she does it when she licks at her legs or feet a lot - then I start wondering about hair stuck in her throat.
I will ask the vet about it again and mention the heart thing at our next Spring time visit.
Do you know how they detected the fluid? Did they notice that just by listening to her heart? or was some other test involved?