GPL and Steam:
In order to test what GPL-licensed games are available on Steam Store and how it is implemented, I've installed Steam and installed Blender and Battle for Wesnoth.
In these two examples (most likely dosbox too), It seems there are literally zero functions of Steam in use except the application running inside Linux SteamOS, the auto-updating, possibly usage-stats generation, for which the software possibly wasn't modified.
supertuxkart intended to implement some of the multiplayer services and possibly drivers, so time will tell if they succeed in using the Steam API.
I wonder about Steam having to comply with GPL however. As far as I know if one distributes a binary, one is obliged to also distribute the source, possibly against a fee. That should be easy for them as they can just mirror the sourcce code and charge a fee. But perhaps they also modified the source code of Blender, Wesnoth or DOSbox in order to have the app compatible with or improved for Steam? If they distribute the binary, they'd have to distribute these modifications as well (if they exist) or ask all copyrightholders to relicense for Steam use, unless I got GPL wrong.
Battle For Wesnoth on Steam:
May 2018 Version 1.14 entered Steam Greenlight:
Wesnoth developers considered Steam logins, but currently the software uses only wesnoth forum account for multiplayer lobby logins.