The gulden was the currency of the Free City of Danzig from 1923 to 1939. They were replaced with Reichsmark at the onset of World War II when Wehrmacht occupied the Free City of Danizig and annexed it into the Third Reich. 
Guldens were exchanged at favorable rate to futher boost enthusiasm of local German population (95% of Danzig citizens). As a result very few Gulden banknotes remain to date making them very valuable. Value of a well preserved banknotes:
~$1,500 for the 20 Guldens
~$2,500 for the 50 Guldens, and...
~ $10,000 for (the most rare) 25 Guldens!
 
Krause Papierwerke offers the following Gulden banknotes
20 Guldens from 1932
25 Guldens from 1931
50 Guldens from 1937
 
For added realism in look and touch the pemium banknotes (printed on 100% cotton paper) can be ordered (recommended!). 
 
Make sure to visit Money Page of Krause Papierwerke for more banknotes of the period!