The Gairdner Child is a version of The Gardener, which Professor Child assigned his number #219. He printed three texts, one of which was a fragment. No indication is given in the DT file as to Ewan MacColl's source, assuming that it is the text he used, nor as to where the two tunes given came from, or which one he used. There is another text in this thread: clarification: The Gardener
People do get confused by the Lady Maisry business; this is a common alternative title for several quite different songs, including Bonnie Susie Cleland, The Maid Freed From The Gallows and Mary Hamilton. It is often a mistake, particularly when dealing with traditional song, to assume that a shared title necessarily implies any other connection. As Bruce pointed out above (over a year ago), Lady Maisry is most usually Bonnie Susie Cleland (Child's number #65). He wasn't suggesting any connection with The Gardener; quite the opposite, in fact.