My English teacher gave this expression to me at the lesson. She told that she had been taught this by English students who studied at Cambridge University. They said that even among English people not everybody knows it. Is it true?
My teacher told me that ‘giggling girty’ means a girl who laughs much. Is this expression used in modern English? And where did it came from?
There are a lot of expressions where an adjective is linked with a personal name. In you expression the name is Gertrude, shortened to Gertie. Usually the adjective has the same first letter as the personal name. I can only think of one at the moment describing someone (male) who is a bit confused - ‘dreamy Daniel.’ I have heard the expression you mentioned.
I think the reason hardly anyone knows it is because Gertrude is now a very rare name for young girls or women. So now if we heard “giggling Gertie”, we would immediately think of an old lady, rather than a giggly young girl.