Yes, but as long as:
1) the dev doesn't use the CEG ("copy-protection") component of Steamworks, and
2) the game folder can be zipped, copied, moved, and installed on a non-Steam computer (as I've seen with a few games)
then that meets your "You are in no way reliant on a service to reinstall it, access it or otherwise" criteria.
At that point, Steam and GOG are equivalent, except that
1) you need to install the Steam client for the initial download (possibly annoying/immoral/etc, but not DRM), and
2) Steam can take away your right to re-download it (although again, as long as you've downloaded it once, you can re-install at will).
The latter is DRM, but I'm not sure why Valve would take your game away. To me, that means it's possible for Steam to sell games that are "virtually" DRM-free. Yes, if you buy one of these games, never download/install it (or have an HD crash), AND Valve decides it doesn't want you to have the game anymore and removes it from your library-- then you're caught in a DRM scenario that couldn't happen with GOG. But that's a pretty minor edge condition, no?
Steam/Valve and GOG share the DRM-ish trait that either could go away and move to some Mediterranean island (oh wait, they're already there ) and you wouldn't be able to re-download.