From looking at it, it seems that it doesn't - that it is running the app in the server and updating the screen in HTML5 canvas. It would be cool if it compiles the code to Javascript though... so that you get complete client side platform independent apps out of the current entire Gnome repository!
GTK itself will probably never do that, because it doesn't include any sort of language-processing code (as I understand it). Adding code to GTK that could translate programs between languages would be a huge increase in complexity and scope.
However, a project like GCC could certainly do that. If someone really wanted that to happen, they could add a Javascript code generator to GCC and pair that with GTK's HTML5 backend to produce something like what you want.
From looking at it, it seems that it doesn't - that it is running the app in the server and updating the screen in HTML5 canvas. It would be cool if it compiles the code to Javascript though... so that you get complete client side platform independent apps out of the current entire Gnome repository!