There are 7 trappist beers (6 Belgian, 1 Dutch).
From Gent, your best bets are Westvleteren, Westmalle or Chimay. None isare easily reachable. To Westvleteren you get the train to Poperinge, for Westmalle Antwerp is your best bet and for Chimay you need to go to Charleroi. From the stations onwards you are depending on either a taxi or the (infrequent) buses. Remember that trappists are typically brewed in utmost silence requiring secluded areas ;)
Personally I think the trip is not worth the effort. In passing, surely soperhaps, but as a specific destination, I am not so sure.
What you might want to consider is Watou, which is very nice small town close to the French border. It has the St Bernardus brewery
If it's all about beer, Gent has a lot to offer. My favourite one is here.