All right, here's what you could do:
- Create two event pages for the airport; one without a condition for the event to take place and the second with a switch as condition for the event to run. Let's call this switch "A".
- On the event page of the Gym Leader where the player receives his/her badge, turn the switch "A" on. If the player defeats the Gym Leader, he/she won't only receive a badge, but the second page of the airport event will also be activated.
On the first page of the event you can use a Show Text command to show a message box saying that the player can't access the airport yet because it's under construction or something. On the second page you can let the player use the airport.
I hope it's clear now, and if you have any questions, feel free to ask them and I'll see what I can do. Also, happy new year ;)