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So, I have the Steam versions of all of these, simply because they were available there long before they appeared on GOG.

Aside from the part where, of course, the GOG versions are DRM-free, is there any reason to re-buy these on GOG? Have they benefited from any GOG-only, 'run on modern machine' type tweaks? Are there any must-have extras included with the GOG versions?

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No DRM, obviously (excluding CD-key for multiplayer). You can take the installer anywhere you want and install it.

Each game comes with a soundtrack (MP3/FLAC) and concept art, wallpapers etc.

There isn't exactly any major reasons for purchasing from GOG if you already own the game. It's mostly up to you.
as above, but ill add that due to that, its a lot easier if you're messing around with different mods as they often require a clean install