How do I create galleries like the ones at the Mono demo?
These galleries are generated by Jetpack Plugin. Just enable the “Tiled Galleries” module and you are set to go! For more information on the Gallery module and how it works, you can have a look here: http://jetpack.me/support/tiled-galleries/
If you want the gallery to open in a carousel as well (like it is shown at the demo), make sure you enable the carousel module as well!