The Great Market is a building.
upgrade => Fair
This building is an improvement of the Market. It therefore requires the latter to be built.
To construct a Great Market, it is necessary that none of the adjoining provinces constructs a Great Market or a Fair itself. If one of your neighbours finishes the Great Market before you, while you have already started, your work is cancelled and you lose all invested means!
This building requires 9300 man-days
The construction of the building requires 2 political points, 3,000 gold and 40 wood.
This building raises the taxes in the province by 0.2 points, including the 0.1 bonus from the Market