The Russell 3000 index will go through its annual rebalancing on June 25. One of the additions will be GGP. This will increase demand for the shares as funds and ETFs that mimic the index will need to buy shares of GGP. This is just the beginning of the demand for GGP from indexes, funds, and ETFs.
Update: The initial post said Russell 1000 but it is actually Russell 3000.