I activate GSX at the gate and there are no vehicles present even though it's active (it still asks me to open doors) Also I hear people boarding but it only takes one second and it says boarding completed.
You don't say which airplane you used, because that looks like an issue with trying to use an unsupported airplane.
You don't say which scenery you used and, of the scenery, which gate you used.
I've noticed that this has been happening ever since I installed both FlightBeam KIAD and KDEN.
It's not possible that just installing the sceneries would cause this. Unless you installed the sceneries without an internet connection, or with a program like a firewall/antivirus that was blocking the ability of the Flightbeam installers to download the most updated files, so you might have installed outdated files.
Previously addon manager was causing the FlightBeam buildings to disappear(not because of any anti virus software because I do not have one and also they install fine when GSX isn't around) but I got it fixed and now I am having the GSX issue.
The Addon Manager cannot cause the Flightbeam buildings to disappear, because they are obviously totally and entirely compatible. The Flightbeam sceneries use exactly the SAME version of the Addon Manager and they download it from OUR site, and it's the same used by GSX, of course.
In fact, it's exactly the opposite, if you DON'T have the Addon Manager, all sceneries (FSDT and Flightbeam) will miss their buildings.
The solution to being sure you have all files updated, is to download the CURRENT GSX installer, and install it last. This way, even if some kind of software blocked the download of updates during the installation, since the full GSX installer is always kept current with all the latest files, it should work under any situation.
Of course, you must be sure you are using the current installers from Flightbeam too, but still install the full GSX as last.