The value or price of the Gulden arises from the game of supply and demand on the Gulden exchanges. If there is a lot of demand for Gulden the price will rise and if the supply exceeds the demand the price will fall.

We now have two exchanges that matter, namely:
Stex: https://app.stex.com/en/basic-trade/pair/BTC/NLG
Bittrex: https://www.bittrex.com/Market/?MarketName=BTC-NLG

At both exchanges, Gulden is traded against Bitcoin but at Stex you also have the option to trade Gulden against Novo https://app.stex.com/en/basic-trade/pair/NOVO/NLG
A DEX is currently under development
All exchanges have a different price at the same time. This is because there are always small differences in supply and demand between the different exchanges. So there is no such thing as the price of Guilder.

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