Jared15......open your aircraft cfg, file. On the top paragraph you'll see all the info FS reads about the plane. Title, sim, texture. etc. What you need to add are these two lines, just copy and paste them in the top parragraph of the config file.
atc_parking_types=gate
atc_parking_codes=EAL
I fly for Eastern Airlines Virtrual so our parking code is EAL...You said you flew a Delta 737-800....Then change the code to DAL for your airline OR what ever airline you want in there. You can also change the parking types too! If you have a cargo flight, you can change the parking statis to CARGO, or GA, ramp or all of the above. What this does is tells FS you have a gate, ramp, or cargo parking spot and will send you there or close to it IF the gates are open!!!
Now if you fly for a VA or airline that isn't in the AFCAD then we need to change something in the AFCAD file. But thats something a little harder, so for now just add the two lines, and do that with all you airplanes.
Hope that helps,
Cheers,
Doug