Law /
Poland's citizenship by descent question. Polish great-great grandfather arrived in the USA as a kid. [76]
1920 citizenship law, which said that anyone born on the territory of what had just become Poland
No, the 1920 law doesn't say that. The law says that it applies to those currently settled on the territory of the Polish State, which your great-grandmother wasn't. As she had left Poland, she had no claim to Polish citizenship.
I wonder if there was any type of document that she would have needed in the US that was issued by Poland?
If you can find evidence of her being issued a document by the independent Polish state after the introduction of the 1920 Citizenship Act, then you might have a small chance. However, it's unlikely - in the US in those days, it was enough to have documents issued at Ellis Island or otherwise.
The odds are that she was never recognised as a Polish citizen. You can try to prove that she was a Polish citizen, but it will cost you a considerable amount of money to put together an application that has a chance of succeeding.
However, there is some good news for you. If you can prove that two great-grandparents spoke Polish and were of Polish ethnicity, Poland will grant you permanent residency, which allows you to live and work in Poland freely. You can't use it for anything other than tourist/business travel to other EU countries, but it's a good option if you want to relocate to Poland.