Hah, a while back I made a Userscript to do exactly that. It basically mimics the 'Windowed' mode on the GB player.
To use it, you need to install the
TamperMonkey Chrome extension. Then click the tampermonkey icon and select 'Add new script'. Copy/paste this
http://pastebin.com/v5ESHUat.
To toggle fullscreen, just press forwardslash (/) on your keyboard.
I've not used it in a while, so it might not work with the new Gamespot live player. I'll test it on the stream later and update if necessary.
EDIT: It does indeed work with the new player.