mindblast: Now, as for Steam, you can run the same game on two different machines, in the same time, as long as they are DRM-free. If they are not, you can't run two steam clients with same logins in the same time. As for GOG games, practically, you can copy them on more systems.
PookaMustard: In addition to that, I think that you need to run these DRM-free games through their executables in order to take advantage of their DRM-free nature and play on as many devices as you want, because if you try the regular Steam way, Steam will complain because you're already logged in somewhere else and won't let you continue.
Careful with these DRM-free games though. They may be DRM-free, but they're still bound to Steam's rules and methods of all their games, backups require Steam to work and the works.
Only if you use Steams backup tool. The drm free games on Steam can be backed up manually and works without steam afterwards. They are not really bound to steams 'rules and methods'... if they did they wouldn't not be drm free.... you only need steam as a downloader for the initial download. I think we have been here before?