The correct file download location is on Mediafire, as it always has been. Nobody has been hacked, and I can see the download page and download the installer just fine.
Mediafire works on two different ways: the free version, where downloads are presented with a Mediafire page (possibly with some Ads, which I cannot see since I have an Adblocker...), and the paid version, where we pay for bandwidth, so users can download directly without seeing any ads.
Of course, the normal situation is to always have the paid version, and there's a feature that automatically recharges bandwidth when it has been exhausted, billing us again for a new chunk of bandwidth. It's normally always enabled but, for some reason (maybe due to the switch from 2017 to 2018), it disabled itself automatically, so the sponsored version of the page came out.
It's now fixed so, all downloads should start directly now.
Throw away all your installers, and download and install only the latest GSX full installer. Vehicle updates installers don't exist anymore, since the new Live Update can update everything, and the Stand-Alone Addon Manager is not required if you are installing the full GSX installer, since it comes with it, of course.