Tough one.
GT's are measured c-c. So, if the seller is quoting 20" it may be a true 18" GT.
In reality, this means 18" c-c and 20" c-t (of the seat tube).
I am 6' and ride a 18" GT.
Here is a pic of my old 18" GT LTS so you can get an idea of the saddle height.