As far as I know, there's no "ore behind the doll besides being a representative 'souvenir' of the Gamemasters program that once existed. All of the information I will provide came from TibiaWiki and from Tibia's website news archive.
The 23rd of August 2010 the role of voluntary Gamemasters came to an end. Due the growth of rule enforcement tools, Cip has determined that volunteer Gamemasters are no longer needed, hence voluntary Gamemaster's were deleted entirely from the game. Some became senators, whilst others became senior tutors.
You see a gamemaster doll. It weighs 15.00 oz. It was awarded in memory of the dedicated service of Tibia's gamemaster team.
All Gamemasters received this doll on August 24, 2010 along with one other item of choice: Dark Trinity Mace, Solar Axe, The Epiphany, Great Shield, Ferumbras' Hat, Rainbow Shield, Snake God's Wristguard or Royal Crossbow. A few ex-Gamemasters that were still active at the time also received the doll.
This is all the information I have read about the doll, at least from official sources.