Swedrami: As reported here, there apparently just recently was an issue with the standalone multiplayer mode for Destroy All Humans! 2 disappearing but then popping up again in the library for people that got it for free alongside the pre-order for DAH! 2.
Maybe happened to you as well?

Timboli: OP - Have you run the GOG 'refresh' option?

With all the changes lately, it may well be your profile's stored values need updating using 'refresh'.

Aside from there being that discrepancy of one game, depending where you look (menu or library), no-one else has admitted to your issue. So likely just a temporary glitch.

Timboli: Well that is weird, and I have certainly never seen anything like that.

Are you the only one on your PC that logs onto GOG or has an account at GOG?

Have you gotten GOG games from elsewhere maybe? I only have the one via Humble, Seven: Enhanced Edition, which every time I use the 'refresh' option or GOG changes something, gets sorted to the top of my library. Might especially have an impact if you have gotten GOG based DLC elsewhere.

Then again, if you use Galaxy, might be related to that, especially if you have imported games from other stores.