Gate cost is 75% of a rented gate. You can expand it (build more gates) at will; it's expensive but you don't have to fight for free gates. Unused slots do not force you to give back gates (unless you still have some rented).
They slow down (if only a little) growth for larger airlines. 25 gates is the maximum you can rent at an airport. After that you must build a terminal. On the other hand, this also prevents a single airline from hogging all gates at an airport.
Terminals are very expensive. There's a minimum of 5 gates to start one. Gates can not be returned (unless you email staff).
