If you have spare gates, you can return them on the View Gates-page. Gates that you have rented, but aren't used, will still cost you money, so it might be a wise decision to return them.
Please note that you can only return gates if all the ten slots for that gate are available.
Navigate to the View Gates area first (Gates, then click View Gates). This will give you an overview of all the gates in your possession. If there are airports where more than 10 slots are available, a checkbox will appear next to that airport. Click the checkboxes of the airport (or airports) you want to return gates at, and once you're done, click the Return button on the bottom of the list.
Now, for each airport you selected, choose the number of gates you want to return. The dropdown menu's will only allow you to choose the maximum amount of returnable gates, so don't be afraid you'll return too many. When you're done for all airports, click the Return button again to confirm your selections and return the gates.
Note: You will not be refunded any paid rent, even if you just rent a gate for a few minutes (whether intentionally or accidentally), so be wise in your decisions!