As far as I know, Google Fonts are free and open-source, so you can use them freely without any further references to Google Fonts or Google.
On the Google Fonts site they state:
All of the fonts are Open Source. This means that you are free to share your favorites with friends and colleagues. You can even customize them for your own use, or collaborate with the original designer to improve them. And you can use them in every way you want, privately or commercially — in print, on your computer, or in your websites.