I've already received an updated AFCAD + GSX .INI file for KIAD from Flightbeam, and it's already up on the self-update server so, anyone installing KIAD (or the Stand-Alone Addon Manager, or GSX) from this moment on, will automatically get the updated files, without having to download anything extra.
If you already downloaded the KIAD installer, there's no need to re-download it, just running it again, will trigger the update.
The only thing to note is, IF you already started customizing the GSX file before we posted this fix, your modified KIAD.INI file in your %APPDATA%\Virtuali\GSX folder will take precedence over the KIAD.INI in the Flightbeam\KIAD\scenery folder, so you will not see the updates, since the .INI file in the %APPDATA% folder takes precedence over everything else. So, you'll have to either remove your custom file OR fix the missing Pushback yourself. This only concerns those that already customized GSX at KIAD today, if you haven't, don't worry about anything.