Yes. You can build a terminal. That costs A LOT of money. Maybe a young airline does not have that kind of money available. So, on to second alternative...
Keep checking, as gates do become available from time to time. Most often at month change but they can appear anytime, really. If another player already has a terminal at your airport (see if someone has already more than 25 gates) then, each time he/she extends that terminal by 10 gates, one rented gate must be returned.
What world and airport are you based at?