Using gold as a currency if it is primarily circulated domestically is not much different than if you were just dealing with a barter system of trading goods for goods.
The main issue, as I see it, is circulation and trade outside your territory. For instance, who are Islamic State’s main trading partners? What goods are they selling internationally? Are Turkish oil smugglers going to have to pay with gold dinars or will IS continue to accept liras or dollars?
If IS have a substantial impact on international commerce and they refuse to accept any other currency besides their own, definitely it will have an impact. If they are buying in dinars, and other economies accept them, while they continue to sell in liras or dollars; I don’t think it is viable, and obviously, their own gold reserves will be depleted.
There has been limited use of gold dinars in Kelantan state in Malaysia (approximately the same size as the area under IS control), and the global financial system hasn’t really noticed.
If a trading bloc can be established in which gold dinars are the sole currency of buying and selling (as was actually proposed by Qaddafi many years ago), obviously the impact on the global financial system would be massive.