If you are presenting, it makes more sense to use the 2nd screen for your audience , and your laptop screen for yourself, your desktop clutter, and your awful desktop wallpaper.
Once Mirroring is switched off applications like Keynote (and even PowerPoint) will automatically 'know' this and and present this way automatically. You might have to adjust some preference settings if it doesn't do this automatically.
But what if you're using Google Presentations because you want to collaborate on a presentation because you have several contributors? Well for starters, BRAVO for using ICT the way it SHOULD be used, now everyone can work on the same presentation, and you can avoid all the headaches with people trying to merge PowerPoint and Keynote presentations, different slides/sizes/versions/templates ... Using an online presentation tool is a much better way to manage this (Prezi, and SlideRocket are great as well).
... and when you're all finished, download the Google Presentation as a PPT file, open it in Keynote, make a few tweaks to font size, polish up rough edges etc. And run it as a Keynote.
But you can run a Google Presentation on a 2nd screen, like Keynote, here's how:
First make sure you are utilising the 2nd screen you have to have a 2nd screen plugged in for this to work. When you've done that, you're ready to proceed...
Now make sure that Mirroring is OFF: System Preferences > Displays > Arrangement (tab) > Mirror displays should be unticked...
Drag the 2nd display where you feel it makes most sense to the left, above or to the right of your Mac screen (that's the one with the white bar at the top) - in this example it's to the left.
Now to control anything on the 2nd screen just move it to where you positioned it, in my case I just drag it off the left of my Mac screen and it appears on the 2nd screen, usually via a projector.
Now You're ready to go. Open the presentation in your browser as normal, and view > start presentation.
Now grab it by the menu bar at the top and drag it over to the 2nd screen... Don't use full screen, instead use the green maximise button - and maybe drag the edges to fit...
Almost there... on the 2nd (presentation) screen, click on the gear wheel and choose 'Open speaker notes'
Now grab that window and bring it over to your Mac screen ...
Done, now you can use the speaker notes to control your presentation, and view notes on your Mac screen, while the presentation displays on the presentation screen.