Cal. Code Regs. tit. 18, § 16563 – Information Confidential.

(a) All gift tax returns and other information and records acquired in connection therewith by the State Controller or any of his employees are confidential in nature and, except as may be necessary for the enforcement of the Gift Tax Law, cannot be divulged to any person other than the donor or his duly authorized agent, or any local, state, or federal tax official.
(b) If the Controller has received the information to be divulged from the United States Internal Revenue Service, the person to whom it is divulged shall be advised of its source and that the confidentiality afforded thereto must be the same as if it were received directly from the Internal Revenue Service. Such information may be divulged only (1) to representatives of the Internal Revenue Service; (2) to the State Board of Equalization and the Franchise Tax Board; and (3) in judicial proceedings for enforcement of the California Inheritance and Gift Tax Laws.
Note: Reference: Section 16563, Revenue and Taxation Code.