The 'View Gates' page will give you an overview of all the gates you have currently rented, and also the amount of slots available at the airport. To go to the 'View Gates' page, navigate to Gates and click View Gates.
When you click the View Gates'-button, the default view will give you an overview of all the gates you have rented all over the world. You can also choose to view all gates sorted by continent. To do this, click the dropdown menu in the righthand top corner of your screen, and select By Continent.
In the default list (which is the same as sorting by All Gates from the dropdown menu), the first airports shown are your focus cities. They are printed against a dark gray background. After that, all the other gates you've rented show up in alphabetical order, based on the actual name (not the airport code) of the airport.
Also shown is the utilization percentage of the gates, which is the amount of available slots you actually have in use. After that, it shows the total gate value of your gates at that airport. The next column says Tagged. If you have rented gates from an airport that you don't actually use, they may get tagged automatically by the game. At the end of the month, you will likely recieve a message saying you have to return those unused gates.
If you have more than 10 slots available at an airport, the game will give you an option (via a checkbox) to return the gate. This will save you money at the end of the month, seeing as you don't have to pay for the unused gate anymore.