1) Is there any dispenser trucks or big fuel trucks than the small ones I have met till now?
There's only a single variant of the fuel truck right now, it will surely be expanded in the future.
2) I am using Fly Tampa Athens airport and I can use the jetways when parked.
Is this a question ? It's not very clear...
In any case, this has been discussed too many times already on the forum, and of course it's clearly indicated on the manual too.
Everything related to jetways operation is entirely unrelated to GSX, the ONLY thing that GSX does, is checking if your parking has a jetway or not, so it will display front stairs (or not) and will remind you (or not) to operate it.
So, if the scenery you use is a proper FSX scenery with animated jetways (like all FSDT, latest Flightbeam and UK2000, for example), they'll continue to operate as they did before. But if the scenery you used a fixed/static/fake jetways, GSX won't do anything to animate them.
I miss any updates or vehicle packages?
You can't miss any updates, because the Live Updater will notify you, and you can also force a check using the Couatl menu. All the latest Vehicles are always included in the full installer as soon as we release them so, if you downloaded the GSX installer recently, it already contains everything updated. If you don't, there's a link on this forum to download the update for Vehicles only.