From the airport
From Brussels Airport for Gent St Pieters Station:
Bus: a shuttle bus runs about ten times back and forth between Brussels Airport and the Gent-Sint-Pieters Railway Station. There, you can take tram line 1 or 3 to the historic centre. The cost is €9,99 per person and per trip.
Train: There is a direct line to Gent-Sint-Pieters station from Brussels Airport-Zaventem. The cost is €18,80 per person.
Taxi: It's a 55 minutes trip and will cost around €160-€170 depending on traffic.