California Code, Probate Code - PROB § 20210

(1) To the extent the transferor in a written instrument transferring property specifically directs that the property be applied to the satisfaction of a generation-skipping transfer tax or that a generation-skipping transfer tax be prorated to the property in the manner provided in the instrument.

(2) Where federal law directs otherwise. If federal law directs the manner of proration of the federal generation-skipping transfer tax, the California generation-skipping transfer tax shall be prorated in the same manner.