Gordon BBBM is Claude Simpson's The British Broadside Ballad and its Music.

In The Ballet Lute book it starts about half way down page 101 (using the pencilled page numbers - it's page 105 of the pages on Trinity's site William Ballet's Lute Book") and finishes on the second stave on p103, where you can see the name at the end. (You might find it easier to download the pdf)

Here's another copy of Sellenger's Round for lute: Cambridge MS Dd.3.18 - Sellengers Rounde - lute tablature (png image). There are other lute versions I think.

The origin will almost certainly be unknown, all we have is where it was written down or referenced. It was used for a song in John Pickering's interlude Horestes 1567, though I don't know if that's the earliest reference.

(Much the same - in terms of unknown origin - applies to Kemp's Jig in your other thread.)

Mick