In States which do have laws to mandate and/or fund some level of gifted education, this has occurred due to parental organization and legislative advocacy. Parents may wish to gather other interested parents (and professionals such as psychologists, OTs, teachers, administrators, and the local NAGC State affiliate), organize an outreach, compose compelling impact statements, research legislation from other States, and contact legislators with statements describing how legislation would benefit the residents of your State.

That said, many parents have described "gifted programs" which consist of curriculum one grade level advanced, which does not present an authentic learning opportunity for many gifted kiddos.

To encourage parents that the effort is worth it, whether helping to craft legislation, or advocating for gifted services which match the program to the child rather than matching the child to the program... I'll share this article from the Davidson Database, titled Gifted children: Youth mental health update, by Julia Osborn, 1996.

There is a section about midway through the article subtitled Special Needs of Gifted Children, which describes these four needs in detail:
A. Need for a challenging education.
B. Need for "true peers."
C. The need for responsive parenting.
D. The need for adult empathy.
In the clinical experience of the author, a range of behavioral problems (from daydreaming to school refusal) have resulted when the school curriculum was not sufficiently challenging.

The next section of the article, subtitled Giftedness and Self-esteem, highlights findings of a study by Miraca Gross:
In her study of exceptionally gifted children, Gross has reported that the self-esteem of exceptionally gifted students tends to be significantly lower than the self-esteem of average students, especially when the school is unwilling or unable to allow them access to other children who share their levels of intellectual, oral and psychosocial development. Thus the gifted child is placed in the forced dilemma of choosing to minimize intellectual interests and passions for the sake of sustaining peer relations or of pursuing intellectual interests at the cost of becoming socially isolated in the classroom. As Gross poignantly added "The gifted must be one of the few remaining groups in our society who are compelled, by the constraints of the educative and social system within which they operate, to choose which of two basic psychological needs should be fulfilled."

While I fully understand the OP's question, at the same time I want to point out that differentiation can be a meaningless buzzword which only indicates something is different... it does not imply that curriculum, placement, pacing, etc are better suited to the child or are in the child's zone of proximal development (ZPD).

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NOTE: The article "Gifted children: Youth mental health update" by Julia Osborn, 1996, published by Division of Child and Adolescent Psychiatry, Schneider Children's Hospital, is backed up on WayBack Machine, Internet archive.
Links:
1) article -
https://web.archive.org/web/20200112034938/http://www.davidsongifted.org/search-database/entry/a10170
2) list of archive dates/times -
https://web.archive.org/web/*/http://www.davidsongifted.org/search-database/entry/a10170