This article is from the Bee Gees FAQ, by David Garcia firstname.lastname@example.org with numerous contributions by others.
The tuning is what he calls a "Hawaiian" tuning, consisting of
guitar strings tuned as follows (from lowest note to highest
D A D F# A D
Thanks to Greg Luther for tracking this down on "a web site
that has a few selected Bee Gees songs for guitar." The address
of that web site:
Maurice, incidentally, uses standard guitar tuning. I like
it that way -- the contrast between the two guitars came out very
nicely on songs such as "Come Home Johnny Bride" and on their
medleys on the "Midnight Special" TV appearances.