From MaGog's work-in-progress logs:
NOTE! CHANGED Cannon Fodder, os: Windows (XP, Vista, 7, 8) and Mac OS X (10.6.8 or newer) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Consortium: Master Edition, The, os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Dracula Trilogy, os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Dust: An Elysian Tail, os: Windows (Vista, 7, 8) and Mac OS X (10.7.0 or newer) ***TO*** Windows (XP, Vista, 7, 8) and Mac OS X (10.7.0 or newer)
NOTE! CHANGED Ether One, os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Ether One Deluxe Edition Upgrade [DLC], os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Ether One Deluxe Edition [Premium], os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED FRACT OSC, os: Windows (7, 8) and Mac OS X (10.7.0 or newer) ***TO*** Windows (XP, Vista, 7, 8) and Mac OS X (10.7.0 or newer)
NOTE! CHANGED Giana Sisters: Rise of the Owlverlord, os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Gods Will Be Watching, os: Windows (Vista, 7, 8) and Mac OS X (10.8.0 or newer) and Linux (Ubuntu 14.04, Mint 17) ***TO*** Windows (XP, Vista, 7, 8) and Mac OS X (10.8.0 or newer) and Linux (Ubuntu 14.04, Mint 17)
NOTE! CHANGED Gods Will Be Watching: Special Edition Upgrade [DLC], os: Windows (Vista, 7, 8) and Mac OS X (10.8.0 or newer) and Linux (Ubuntu 14.04, Mint 17) ***TO*** Windows (XP, Vista, 7, 8) and Mac OS X (10.8.0 or newer) and Linux (Ubuntu 14.04, Mint 17)
NOTE! CHANGED Gods Will Be Watching: Special Edition [Premium], os: Windows (Vista, 7, 8) and Mac OS X (10.8.0 or newer) and Linux (Ubuntu 14.04, Mint 17) ***TO*** Windows (XP, Vista, 7, 8) and Mac OS X (10.8.0 or newer) and Linux (Ubuntu 14.04, Mint 17)
NOTE! CHANGED PixelJunk Shooter, os: Windows (Vista, 7, 8) and Mac OS X (10.6.8 or newer) ***TO*** Windows (XP, Vista, 7, 8) and Mac OS X (10.6.8 or newer)
NOTE! CHANGED Quest for Infamy, os: Windows (Vista, 7, 8) and Mac OS X (10.7.0 or newer) and Linux (Ubuntu 14.04, Mint 17) ***TO*** Windows (XP, Vista, 7, 8) and Mac OS X (10.7.0 or newer) and Linux (Ubuntu 14.04, Mint 17)
NOTE! CHANGED Schein, os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Sir, You Are Being Hunted, os: Windows (7, 8) and Mac OS X (10.7.5 or newer) and Linux (Ubuntu 14.04, Mint 17) ***TO*** Windows (XP, Vista, 7, 8) and Mac OS X (10.7.5 or newer) and Linux (Ubuntu 14.04, Mint 17)
NOTE! CHANGED Strike Suit Zero: Director's Cut, os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Sudeki, os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Tower of Guns, os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Windforge, os: Windows (7, 8) ***TO*** Windows (XP, Vista, 7, 8)
NOTE! CHANGED Wing Commander 5: Prophecy Gold Edition, os: Windows (Vista, 7, 8) ***TO*** Windows (XP, Vista, 7, 8)
I find it rather unlikely that so many games have suddenly developed XP and Vista compatibility out of the blue (sorry, out of the grey). As it now stands, every single game on GOG is XP and Vista compatible. Note that these changes are not reflected in the games' system requirements (e.g. Windforge still reads "Minimum system requirements - Windows 7 / 8...").