The original meaning of mere was pure, true. And Gertrude is probably using it that way – as in, this is real madness. This is pure madness. This is true madness.
Our contemporary sense of mere is almost its opposite. We read a line like this as “This is only madness. This is just madness. This is inconsequential madness.”
I think I said it this way when I played this part. But its original sense makes much more sense.
And apparently both meanings sat side by side for a while before the true, pure sense faded away.
And it seems there was a sort of middle ground meaning as well – or a bridge. Maybe it’s how the word came to mean almost opposite things. Apparently, it also meant glimmering and shimmering, which is easily connected to fairy gold and glamouring. That is, something that appears to be true but isn’t, really.