With launch screen, you mean the new Start screen? All Start menu shortcuts automatically appear there (except if they appear to be uninstallers and such, but those are still searchable and can be added manually). So if you tell Steam to create Start menu shortcuts for a game, they automatically appear on the Start screen.
If Steam can still trivially get its third-party-bought programs onto the metro-styled start screen, the popular reporting on that situation was off.
And I can't even find a story where Gabe himself actually expresses an opinion on, or concern about, the MS App Store. His comments seem to entirely revolve around Metro being bad for desktop usability and Windows 8 not selling. So the popular reporting that Gabe was specifically upset about the store (via it's limitations) seems to have all been bullshit/editorial.
Upon digging deeper, it seems his concerns about Windows 8 aren't materially different than his comments about Vista (via complaints about DX10 being Vista-only, as Vista wasn't selling).
If it were about the app store, why does Gabe have nothing to say about the Mac App Store?
In my poking around on this, I saw plenty of assertions that this was all about the app store, but nothing is ever attributed to Newell except his lament that the usability is off and the sales aren't there.
Because Mac and iOS gaming doesn't represent such a large share of their business. The doom and gloom doesn't make sense otherwise, Steam functions perfectly well on Windows 8, as do all other desktop, non-"Modern UI" apps, so there's no other conceivable reason for him to get irate.
> "The doom and gloom doesn't make sense otherwise"
But the 'doom and gloom', from Gabe's mouth, doesn't seem to be any more extreme than his stated position on DX10/Vista. As far as I can find, the press are the ones taking his comments to extremes, reading into them about app stores, etc.