Cal. Revenue and Taxation Code - RTC § 16710. Section 16710.
(a)
A tax is hereby imposed upon every generation-skipping transfer in an amount equal to the amount allowable as a credit for state generation-skipping transfer taxes under Section 2604 of the Internal Revenue Code.
(b)
If any of the property transferred is real property in another state or personal property having a business situs in another state which requires the payment of a tax for which credit is received against the federal generation-skipping transfer tax, any tax due pursuant to subdivision (a) of this section shall be reduced
by an amount which bears the same ratio to the total state tax credit allowable for federal generation-skipping transfer tax purposes as the value of such property taxable in such other state bears to the value of the gross generation-skipping transfer for federal generation skipping transfer tax purposes.