Heroes of Might and Magic 3 Complete (Steam has HD version without expansion)

The problem is that Valve has used some pretty unsavoury lock-in tactics through its Steamworks API that discourage developers from releasing on other platforms. If well-designed in a low-level development environment (i.e. coded in C or C++), games can work just as well outside of Steam as with it, but the API is designed to reflect the fact that developers will frequently take the path of least resistance, which means that elegantly handling the absence of the Steam service involves extra work, and Valve does not actively enforce this.

A number of Steamworks implementations in widely used middleware solutions for example require Steam to be running when the engine initialises, and for developers using higher-level solutions like GameMaker, RPG Maker or Unity, ensuring that the game doesn't simply exit with an error message lamenting the lack of Steam service can be problematic when the middleware itself causes the game to exit with an error message.

GOG lends itself particularly well to older games because these are usually hacked binary releases which do not allow the Steamworks API to be implemented.

The Galaxy API, as far as I can tell, has the same problems, but I expect that GOG's QA requires that Galaxy API users implement a failsafe that enables games to run outside of the Galaxy client, similarly to the way that Sony, Microsoft and NIntendo's QA procedures require that (fundamentally offline) games on their platforms can be run offline without the game throwing up an error.

There are some, but they are outdated.

I feel this should all be prefaced by 'If a developer opts to design their game with Steamworks', which admittedly many do, because there are a lot of cost cutting benefits to them for using an already existing framework.

While I agree using that framework probably makes it more of an effort to also design your game without that framework (since they probably skipped some basics since they didn't need to develop those since Steamworks already handled those), it's the developers choice not to put those costcutting measures into developing a DRM-Free version as well, which they're perfectly capable of doing.

The issue there is probably not that it requires some more effort, but moreso that it's putting in (at that point) extra effort for a relatively small percentage of your potential userbase, rather than putting that same effort into making that same game better for the majority of your userbase.

TLDR, it wouldn't be that big of an issue if more people (exlusively) bought DRM-Free.

Also on Desura.

https://en.wikipedia.org/wiki/Cory_in_the_House#Video_game

GOG could use some Cory in the house.

Redneck Rampage Collection.

Speaking of which. GOG.com should rename it to Redneck Rampage: Family Reunion instead of Redneck Rampage Collection.