Just drag the game in question's shortcut into the Win 7 Games Explorer window and that will do it.
I've done that, but all it comes up with is the emulator's S logo. Windows 7 find it normally, but it has no icon or info and is placed under "Unspecified"
EDIT: This only happens with Beneath a Steel Sky
Vista is less picky about how games are registered, I assume the process has matured and become more complete with 7. The GOG installer hasn't been updated for all games yet to register per 7's requirements (which works with Vista, so it's not necessarily "different", just a tighter path to follow).
It happens with other games as well, like Evolva and Sacrifice. The installer doesn't register in a way that 7 likes (but Vista accepts), but those (and some other) games are picked up by Windows' own "legacy games detection" and gets a working icon. This also happens with some games where the installer DO register correctly, like Advent Rising, Republic, Divine Divinity, and Psychonauts (so those games get two working icons, one from the installer and one from Windows detection).