OldFatGuy: I own over 1500 games here..... it would take some time to go over all of my purchases and make sure I other games didn't disappear. Is there any other way to tell?
None that I know of - you'll have to do some "legwork".
First things first
(because this may cause some confusion for some): when you hover over your account name, you'll get shown the number of games you own.
If you then click on "games", you'll get taken to your shelf, where you might see a different number
(IF you have games hidden in your account).
Example: you have 1500 games shown if you hover over your account name, but you only get shown 1478 games when looking at your shelf...that means you have 22 games hidden - which can easily be confirmed.
Simply click on the arrow besides "My Collection (# of games)" to expand the menu.
(you'll get shown "My collection (#), Updated (#), Hidden (#))
If you never bothered to do it, it is a bit of a pain in the ass with so many games, but you'd be well advised to create a spreadsheet with all your
(GOG, Steam, UPlay, Origin, etc) games, that you'll have to keep up to date, of course
(meaning: with every new purchase, you immediately add that new game to the spreadsheet).
One piece of advice: keep the GOG nomenclature, it makes it easier to compare the lists later.
OldFatGuy: Am I missing other games now too?
Does anyone else know of this issue with other games?
Oh believe me: if there were games missing from people's accounts, you would notice.
People would create several threads to let you know...like they did with TES: Arena and TES: Daggerfall.