KIAH being an FSDT scenery, you are allowed to download and install GSX Pro there, and use it there ( same with all the other FSDT sceneries ) for free, you'll have to purchase GSX Pro only to use it on other airports.
So you can just consider GSX Pro as an extra optional download for the airport, even if you don't plan to use it elsewhere. The main thing you'll miss if you don't use it with GSX, are VGDS, and in this airport there's a lot of them ( about 150 ), the rest are GSX-specific things like passengers interacting with the scenery doors at Terminal B and C, passengers walking inside interiors at C, custom Pushback for basically all gates.
