The key you want to edit is: /apps/docky-2/Docky/Interface/DockPreferences/Dock1/Plugins
This assumes GMail is on your first dock (if you have more than one).
The key you want to edit is: /apps/docky- 2/Docky/ Interface/ DockPreferences /Dock1/ Plugins
This assumes GMail is on your first dock (if you have more than one).