That yaml is snap declaration yaml, not snapcraft yaml (which is why I pointed this out to an architect and the requesting publisher, not everyone).
Your existing yaml is close. This is what you need:
plugs:
gnome-3-26-1604:
interface: content
content: gnome-3-26-1604
target: gnome-platform
default-provider: gnome-3-26-1604
The important thing is that you do either:
plugs:
gnome-3-26-1604:
interface: content
or:
plugs:
something:
interface: content
content: gnome-3-26-1604
The latter explicitly declares the ‘content’ attribute of the plugged content interface. This is what must match for auto-connection. The former is a shorthand where the ‘content’ attribute isn’t specified, so the name of the plug is used for ‘content’ (so in that case, the name of the plug must match for auto-connection).
See: