This is not complicated.
I just want to know maybe two or three things.

1. Did anyone ELSE get a 4g+ update for Aliens: Isolation ?

2. Has ANYOne ever completed a download through the GoG decktop game management interface,
and when it was done click Play only to have the Game in question Completely removed from you rig,
oh but wait it gets better ....then the Entirety of the GoG Galaxy program wiped from the drive at That
EXACT moment ?

That right - no sooner did this Aliens: Isolation massive update ( that i can find NO info about anywhere) finish, then the whole works vanished.

I ran some scans, changed my p/w ... WHAT ?!?!?!?!

Input please

1. Check the quarantine section of your antivirus.
2. Check that your drives aren't failing using CrystalDiskInfo
3. Check that your file systems are healthy chkdsk.

According to GOGdb, the game hasn't received any updates. I also can't find anything about it in the what just updated thread. I doubt GOG Galaxy automatically performs a Verify/Repair on installed games, but it's not out of the question.

In the quarentine there's three things from tonight:
is-UTK3B.tmp
is-S6ILC.tmp and the GoG client itself

I'll try the chkdsk. man that's a way-back moment. I remember that and a C: prompt ...hundred'n fify million yrs ago

ty... oh and game directory for Aliens: Iso was still full. 30g. Only the new install of GoG didn't see it among the other "Installed games"

I tried a fresh install of the game and it reported my "local" game to be corrupted. idk

I went for the chkdsk, and one Idk if ssd needs it done or not (my guess is it's fine) but Win10 said immediately it just didn't need it. It reported that it hadn't found anything wrong "already".
Also I'm still confused about the 4g Update ... What about that. It wasn't my imagination. It went to it immediately when I got there to install it. I was really puzzled to see myself but there's no mistaking the fact that GoG was installing it. What the heck was it? weird man - Oh and the 2nd attempt at reinstalling the same thing.
It's saying my Local Copy of the game is corrupted and then giving me an option to Uninstall. So I took it.
But I did that before - how can something that's not there be corrupted?

ty for the input Ice_Mage

There wasn't really a need to bother with the other steps since you found the issue from the start. Open your antivirus and do a manual check for new virus definitions. Then I'd reinstall the game using the offline installer. You can normally download these via GOG Galaxy in the Extras section, but since that's also getting thrown in quarantine, you'll have to use a browser.
If that also gets quarantined, then get in touch with the antivirus vendor about what is hopefully is a false positive. The other possibility is that a virus on your system is infecting your programs. Hopefully it's not that.

Galaxy itself got an update yesterda and it's all still working.

The suspicion that your AV put some files into quarantine sounds solid.

What are you using? I heared from other sources (don't use it myself) that Windows defender lately goes completely crazy.

Been a while. Sorry -
At the time I was probably using PCMatic / PCPitstop -
Since then I've eliminated that (thought my wife purchased it for x5 devices lifetime. Honestly from a utility standpoint it's miserable and clunky. No problem of that sort since. Apologies for taking this long.
(side note) we've also implemented battery backup. A hundred bucks for complete peace of mind over rolling brownouts or storm outages. In July I lost (different rig) two SSD in less than 10 seconds, quite literally. A beautiful sunny July day too. If you've not done this, do yourselves a favor. Takes a few minutes.

Be aware that most pc utilities are bloatware or even malware. You can try any number of freebies that do a better job. Mainly because the base functions work, while the paid parts that are not active are usually bloat anyway. Avast, ClamWin and Malwarebytes offer free software and they tend to score reasonably well.