Creates the Government Contract Retainage Act. Defines terms, including that "retainage" means a portion of money withheld from the payment of a contract for a specified period of time to ensure that a contractor or subcontractor finishes a construction project completely and finishes in the manner specified in the contract. Provides that interest shall accrue monthly on the first day of each month on retainage withheld by a governmental unit at a specified rate set by the State Treasurer. Requires retainage withheld by a governmental unit to be paid not more than 60 days after being retained, including interest accrued, unless the retainage was withheld on the last pay period of a project shall, which shall be paid not more than 120 days after being retained. Limits the concurrent exercise of home rule powers.