“God Rest Ye Merry, Gentlemen” is number 394 in the Roud Folk Song Index. The index number identifies the traditional song across its many textual and musical versions, not a particular printed arrangement.
The carol is English and traditionally authored, with origins that remain difficult to date precisely. An anonymous manuscript from the early 1650s contains an early version, while the earliest known printed edition is a broadside from about 1760. William Sandys helped popularize it through Christmas Carols Ancient and Modern in 1833.
The song is often confused with nearby numbers in historical song collections, including its Roxburghe Collection reference, 3.452. That collection number is not the Roud number, which is why 394 is the relevant answer here.
Another common misunderstanding concerns the opening phrase. “God rest you merry” originally meant “may God grant you peace and happiness”; “merry” does not grammatically describe the gentlemen. The familiar minor-key melody is also only one of several historical tunes associated with the carol.