Essentially, you just upload your images, and apply a gallery setting, for instance:
If you want all your galleries to be the same style (for example the basic 3x grid as above), you could also set these defaults under page.gallery in the bottom of your main settings file so you don't have to apply them to every page.
Another reason why things just aren't as simple as before, is also because of all kinds of different devices from ultra-large 2560 px screens to small mobile phones. A single "grid" setting simply is not satisfactory options for a grid to "just work" on ALL screens. The grid:3,2,1 setting essentially means "3 columns on large screens, 2 columns on small screens, and only 1 column on small devices". Wish it could be easier, but for modern websites, we need to maintain modern approaches also.