I've compared it to authentic signatures of his and many of the details seem to match. I'm fairly certain that it's authentic.

Hi drummerboyjac :)

Thank you for your feedback {up} Great Success :P

You have been added to the list for the $10 gift certificate.

{star} [url=http://www.fanmail.biz/80669.html]Barry Gibb[/url] on Fanmail.biz | [url=http://www.fanmail.biz/feedback/Barry_Gibb.html]Previous Feedback(s) Received[/url]

Fanmail 8)

I sent Barry Gibb of the Bee Gees a letter, photo, and SASE on July 5, 2016. Today on October 22, 2016, I received the photo back signed! This is a huge deal for me as I am a massive fan of Barry's work. I was slightly bothered that the bottom half of the signature was smudged in the time it took to send back the photo, but the majority of the signature was untouched, and for that I am grateful. I used the address in the database:

Barry Gibb
Middle Ear, Inc.
Addicott & Addicott
900 North Federal Hwy.
Suite 201
Hallandale Beach, FL 33009
USA

[img]http://i350.photobucket.com/albums/q410/drummerboyjac/IMG_33181_zpstc5dhrie.jpg[/img]