I apologize; you are correct, it gets 8 in gui mode as well. I must have been confused about which window was running which version of emacs while I was experimenting.
But is that the correct value? The bubbles game code clearly wants it to mean 'the number of forward-chars I have to do from the beginning of the line to get here' not 'what column does it visually look like I'm on'.
The doc string for the function doesn't clarify the intended meaning.
If you do m-x bubbles immediately followed by m-: (current-column) it returns 1 in the gui case but a number larger than one (which depends upon the :width keyword of the wide-displaying space), but for the game to work in the character case it needs to return 1.