This is what Lloyd had to say about the song, in the notes to his 1966 record, First Person (Topic 12T118), which also included his definitive versions of Sovay and Reynardine:

"In the roll call of famous musicians the sonorous name of the Bardd Glas Geraint -Geraint, the Blue Bard- occurs.  He was a 9th. century harper of such legendary eminence that when Chaucer wrote his House of Fame he set "the Bret Clascurion" up in the minstrels' gallery alongside Orpheus and similar well-known string-pickers.  That was in the 1380s, some 500 years after the harper's time, but his fame endured for much longer in the English folk ballad named Glasgerion.  The ballad dropped out of tradition long ago, but the story it tells is an engaging one (a modern and more demographic parallel is  Do Me Ama  ) and it seemed to me too good a song to be shut away in books, so I took it out and dusted it off a bit and set a tune to it and, I hope, started it out on a new lease of life."

Typically, he doesn't say whether the melody is his own or a modified traditional one.  I don't think I've ever heard it sung to guitar accompaniment, but it'll do you no harm to work out your own chords.  I can help with the melody, though, and will email you a .midi if you would like.  Bert Jansch and Pentangle used Lloyd's tune, but slowed it down a great deal.

Malcolm