Ok. This seems like the most trivial thing but stupidly it had me stuck for about 10 mins
Once you have chosen a Google Webfont to use in the Site Settings section, it can be found at the bottom of the fonts list when you choose a font (after you have refreshed the design page if it is already open). It will not be placed in alphabetical order with all the other fonts as you might expect.
Could this be changed maybe?
Anyway, I hope this helps.