That is a very interesting suggestion Glenn, but I am not sure as I think that the diamond frame is different, although the shape of the stone is similar.
Here is a rather blurred enlargement of the brooch at Queen Mary's waist.
And an enlarged detail of the central element/brooch in the Gloucester tiara.
Some time ago we had a discussion about the central element of the tiara as a brooch. There are photos of Queen Mary wearing it.