Google drive icon in Finder

Press the command-key and click/drag the Google Drive application onto the Finder window's toolbar. The icon should remain. I normally do this on the right side just before the Search window.

If you mean in the Finder sidebar Google Drive normally adds itself when installed.

Google would have to author a menu extra item that SystemUIServer would allow on that right-side of the menu bar.

If you are talking about the Sidebar, you should be able to drag any folder into it. However, I have no idea how Goolag Drive works, so that may present a problem.

If you actually do mean the Toolbar, hold down command and drag the folder into the Toolbar. Same caveat.