Galleries Settings
To get to galleries settings go to 'Mobile Centre' > 'Settings' > 'Galleries Settings'
Here you can chose where the galleries come are sourced from. They are both ticked as default, but if you don't want galleries from the website to show in the galleries section in mobile centre you can untick Sites. If you don't want the galleries you make in mobile centre to be available to share on the app then untick Mobile.