I think Gladys simply has a hole through the wooden bung in the end of the wind surfer mast that has been adapted to make the gunter yard.
I do know I use strong plastic electric cable ties to attach sails to spars. Simple. Effective. Adjustable. Low friction. Light weight. Cheap. Replaceable. Can be doubled if you want more strength - but its better to have something cheap and replaceable fail than damage the sail or spar. If one fails (not happened yet) then the rest probably hold instead of the whole lot unravelling. May not look "traditional" but no one has noticed yet, especially if black ties are used to match the colour of the spar.
Matthew
ex-Gladys BR20 Gunter