I looked up your state education laws, Title 22, and section 16.32 says GIEP meetings include "other individuals at the discretion of either the parents or the district", so he can't keep your advocate out.
(b) Each GIEP team must include persons who meet the following qualifications:
(1) One or both of the student’s parents.
(2) The student if the parents choose to have the student participate.
(3) A representative of the district, who will serve as the chairperson of the
GIEP team, who is knowledgeable about the availability of resources of the
district, and who is authorized by the district to commit those resources.
(4) One or more of the student’s current teachers.
(5) Other individuals at the discretion of either the parents or the district.
(6) A teacher of the gifted.