I really don't know what's the point of this entire thread. Is not that, if GSX is slightly outdated, it "won't work" or you are prevented to use it. It will work EXACTLY as it was before, JUST with the extra notification that is outdated, that's it.
It seems somebody is confusing the alert from the INSTALLER which says you can't use it to update GSX while the sim is running, with the actual usage in the simulator of the previous version. No issues whatsoever, you can use GSX as outdated as you like, nobody is forcing you to update to USE GSX. You are only forced to QUIT the sim to DO an update!
And yes, it is required, and the FSDT update always refused to start when the sim was running, the only "problem" was it only checked this in FSX or P3D, it was missing it for MSFS, but it was a mistake, it should have.
The general rule, that doesn't have *anything* to do with GSX, so it's valid for every add-on, is that ANY change to any file in the Community folder MUST be done with the simulator closed. Even if lot of GSX is outside the Community folder (so it *could* be updated on the fly), lots of files ARE in fact in the Community folder and, with the addition of Seated Passengers where we need to modify also files in other add-on, it's all the more important the Updater would prevent you from running with the simulator open.
But again, this doesn't have anything to do with your ability to USE GSX if is outdated. You surely can use it, you just can't *perform* the update while the simulator is running, that's it.