It’s one of these though http://www.duplication.com.au/wp-content/uploads/2011/09/4-panel-gatefold-28587.jpg i.e. folded in the middle with another pocket for the booklet. Isn’t it the fold which makes it a gatefold rather than a sleeve? (In this particular I think it’s actually one bit of card which has been rolled into a tube, flattened and then folded in half, but I’m sure there are lots of ways to make the same sort of thing)
Isn’t cardboard sleeve is something more like this? http://www.groovehouse.com/images/products/diy_stamp_sleeve_1.jpg
Musicbrainz’s docs link to https://upload.wikimedia.org/wikipedia/commons/2/2c/Packaging_Paper_Sleeve_with_CD.jpg which is even called paper in the link… Seems like several of the example images leave a lot to be desired.
