Gabriel Piete is a well known minstrel, who travels around to perform in all the theaters in Britannia. Most of the time he can be found in the Conservatory of Music in Britain though.
Gabriel is very often sought out by autograph hunters, sent by Elwood McCarrin, and will agree to give you an autograph, just to get rid of you.
There is a catch though. He will only sign the sheet music for one of his own songs, this should be possible to find in one of the theaters he last visited. Those theaters are The King's Men Theater in Britain, The Performing Arts Centre in Jhelom and Nujel'm Theater in Nujel'm.
Only one of the theaters will have the sheet music you need, which one is completely random. The person who might sell you the sheet music is the Impresario, who will charge 10 gold pieces for it.
Once you have the sheet music, return to Gabriel and have him sign it. You can now return to Elwood to get your last task.
Note: Gabriel will only talk to you if you have been sent to get his autograph by Elwood.