Can I join in? Just to say that I don't think the point at issue matters all that much. But lets have a good old barney about it.
(Oh no! - it's that junk rig bore again. Humour him, lads.)
Categorically, a junk sail has a yard. This looks surprisingly like a lugsail yard. It has a single halyard BUT, Tony, it does have another 'point of attachment' in the form of a yard parrel, which passes from the yard, round the mast, back through a block on the yard, and down to the deck. This bit of control, additional to the halyard, does NOT make it a gaff.
I suggest that a gaff has jaws, at the mast end of the spar. Anything without jaws, Julian, is a yard: with jaws it's a gaff. The number of halyards doesn't come into it (you mentioned wherries, otherwise I would have). The geometry of a gunter mainsail means that one halyard is enough to hoist the gaff (with jaws - IS there such a thing as a gunter rig without gaff jaws?), which doesn't become a yard for want of another halyard.
I think I've made that sufficiently unclear.
Sources -
'Practical Junk Rig' by Hasler and McCleod
'Jack and Jill and the Hermaphrodite Brig' by Jeremiah Bilgewater (not yet published)
'The DCA Four-part Harmony Shanty and Church Anthem book' (refused publication; available in a plain envelope)