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The JUMP card doesn't work on trains outside of the Brussels region.
On the jump information page you can download a map to where you can use your JUMP card.
If you want to take the train (NMBS or SNCB, same company just other names because of different languages in Belgium) from Brussels to Ghent you'll need to buy a ticket.
Be aware, Ghent has 2 train stations. You probably want to go to the biggest one: Gent-Sint-Pieters