35 ILCS 405/4

(a) Imposition of tax. An Illinois generation-skipping transfer tax is imposed on every taxable transfer resulting in federal generation-skipping transfer tax involving transferred property having a tax situs within the State of Illinois. (b) Amount of tax. The amount of the Illinois generation-skipping transfer tax shall be the maximum state tax credit allowable with respect to the taxable transfer, reduced by the lesser of: (1) the amount of the state tax credit paid to any other state or states; and (2) the amount determined by multiplying the maximum state tax credit allowable with respect to the taxable transfer by the percentage which the gross value of the transferred property not having a tax situs in Illinois bears to the gross value of the total transferred property.