So when I first installed GOG Galaxy 2.0 and did the integrations, it reported that I have 406 games on Steam.

gog1.png

I remembered not this number, but the total that was over 500.

Today I noticed on Galaxy 2.0 that the total had dropped.

gog2.png

I'm missing 16 games now, and only Steam has less games.

Steam profile page itself today lists my games as 380, where GOG Galaxy 2.0 says I have 390

Is there any way to find out what has happened and who is right and why?
Is steam taking out games from my library or did GOG Galaxy 2.0 saying I have more than I do?
Why is today Steam saying I have ten less games that GOG Galaxy 2.0 says I have in Steam?

Hundreds of games were banned from Steam just 2 days ago.

https://www.pcgamer.com/1000-games-were-just-removed-from-steam-and-we-dont-know-why/

Banned games can't be detected by the Steam API that GOG Galaxy uses, but they stay in your Steam library. Maybe you own some of them.

I would accept that but for the fact that my Steam profile this morning said I have 380 games.

So which is right?
The 406 that GOG said I have when I integrated Steam for the first time?
The 390 it lists today for Steam?
Or the 380 Steam says I have?

None of the above is guaranteed to be right, because:
1. You could've already owned banned games when you connected your Steam account for the first time. There are also games with limited profile features, those can't be detected by the Steam API either.
2. The reduced number doesn't matter. See 1. why.
3. The number of games as shown on your profile isn't necessarily correct either, because both banned and "profile features limited" games don't contribute to your game collector counts.

The only correct number of games owned on Steam is what the Steam client itself says. Maybe, but you can't know this for sure (see above). OP would have to check this themselves.