Approximate exchange rate was probably around 500 billion papiermark for gulden.
Almost. The exchange rate between Danziger/German Notgeld (Papiermark) and the (new) Gulden was 750 billion to 1 Gulden. This exchange rate was established on the 31st of December 1923.
Now, when it comes to the 10 Pfennig coin, this coin, if real, is an absolutely amazing find! Congratulations to the owner. If you want to find out something about the varieties, here is a page. There are four known varieties and all of them differ in the most minor details.
https://www.notgeld.com.pl/polskie/index.php?co=gdansk&l=pl
The history of these coins is very interesting. The variety with the small ‘10’ has 60 known varieties/combinations and 20 carved dies. The coin that you own has four known combinations, due to the significantly lower mintage (about 124k, compared to 876k with the small ‘10') This high number stems from the fact that the dies were carved by hand, either in the “Danziger Gewehrfabrik” or the jeweller “Stumpf und Sohn” (their jewellery is highly collectible and rare today). The coin was minted because the city lacked a small circulating coin, which negatively impacted day-to-day transactions. So, they signed a treaty with the Weimar Republic, which allowed them to strike these coins.
But, sadly, the projects of Ernst Petersen and Georg Böcker weren't really popular. People didn't really like them, as zinc coins were…well, quite ugly and they didn't last long.
Not only that, the inflation was rising. Very quickly. It only took about a year for these coins to lose all of their value. Example: Sending a letter.
First half of 1921: 60 Pfennig.
First half of 1922: 3 Mark.
First half of 1923: 300 Mark.
So, these coins were basically useless, meaning that banknotes took over.