I travel through Gateway about 3-4 times a year for allegiant air.
Airport is easy to navigate and small. It has always been clean.
There are two dining options inside security--a grill type place and a Paradise bakery. Also a few gift shops.
TSA here is hit and miss- sometimes I fly through it- sometimes it takes 2 hours... I'm not sure what the deal is. I try to get here early due to a scary (almost missed flight experience) once.
I always park in the economy lot $7/day and catch the shuttle to airport-about 3 minute drive. I've rarely had to wait for the shuttle more than 5 minutes.