You are reading it wrong. The key word in that statement is
multiple.
With the free version of G-Force, you have to pick when you download it which media player you want it to work with. So, for example, there's a version for iTunes and a version for MC. You can choose one or the other but not both.
The platinum version will automatically detect any of the players you have installed that are supported by G-Force and install it for them all. Other big differences are:
1. the free version shows G-Force "ads" periodically overtop of the visualization (just things that say "you didn't buy this, upgrade to the Gold or Platinum version by clicking here"). Just to be clear, these are only ads for upgrading G-Force, not ads for other crap.
2. you can't customize the free version using the toolbar, which means you can't use your own pictures or change the font sizes or resolution (as easily, you still might be able to by manually hacking the INI files)
3. Gold and Platinum come with extra-fancy maps and visualizations and whatnot. Platinum also has the standalone app which can be used with basically anything on your computer that makes noise.
4. you don't get the ultra-cool cover art and track titles in MC. This feature is awesome. It basically briefly shows the track info after the song changes (it fades out after a user-defined delay). Here's an example: