California Code, Probate Code - PROB § 20215

(a) If a person is charged with, or required to pay, a generation-skipping transfer tax greater than the amount prorated to that person because another person does not pay the amount of generation-skipping transfer tax prorated to the other person, the person charged with or required to pay the greater amount has a right of reimbursement against the other person.

(b) The right of reimbursement may be enforced through the trustee in the discretion of the trustee, or may be enforced directly by the person charged with, or required to pay, the greater amount and, for the purpose of direct enforcement, the person is subrogated to the position of the trustee.

(c) The trustee or person who has a right of reimbursement may commence a proceeding to have a court determine the right of reimbursement. The provisions of Article 3 (commencing with Section 20220) shall govern the proceeding, with changes necessary to make the provisions appropriate for application to the proceeding, and the court order determining the right of reimbursement is an enforceable judgment.