I don't use Gnome but I had a look at gnome-control-center
and it looks like this is yet another useful function that the Gnome developers in their infinite wisdom decided to do away with.
It looks like you need to go to "Region and Language" => "Input Sources" and there, you add Spanish. Don't ask me why a keyboard layout (or a language) is considered an "Input Source". Anyway, once you've done so, you should be able to switch layouts with Super + Space.