Project author here; let me confirm that Toga only uses GTK+ under Ubuntu/Linux. On OS/X, it uses Cocoa. I've just clarified the text in that section of the website. Thanks for the heads up.

Thank you for the clarification.

It does not use GTK+ on a Mac, it uses Cocoa, so it is fully native. Read the docs.

Hmm, they should update their front page than. It is pretty clear in stating "Toga uses the system native python GTK+3 bindings for display purposes."

That sentence you quote is immediately underneath the heading "Problems using virtualenv under Ubuntu" — it applies only to Ubuntu, where they do use GTK. But on Mac, they use Cocoa, as stated in several places in the docs.