The National Center for Research on Gifted Education (NCGRE) at the University of Connecticut (UCONN) shares Key Findings.

Of the 16 Key Findings listed, 2 stood out to me. These are the 11th and 12th bullet points, and they highlight curriculum for gifted pupils and growth in the knowledge gained by gifted pupils, respectively:
Originally Posted By: NCRGE Key Findings
- Gifted programs seldom focus on core curriculum such as math and reading. Gifted programs have a greater focus on critical thinking and creative thinking than reading/language arts and mathematics.
- Gifted students start ahead in reading and mathematics achievement at 3rd grade but don’t grow any faster than other groups by 5th grade. In some cases, gifted students show slower growth during this period than non-identified gifted students.

In a column by Jill Barshay, published online April 15, 2019 for the Hechinger Report, curriculum findings are summarized this way:
Originally Posted By: J.B. article - April 15, 2019
The survey found that instead of moving bright kids ahead to more advanced topics, gifted classrooms are preoccupied with activities to develop critical thinking and creativity, such as holding debates and brainstorming. The third most common focus in gifted curriculums is to give students more projects and games, so-called “extension activities” that are tangentially related to their grade-level content. Accelerated math instruction ranked 18th on a list of 26 items that gifted curriculums could focus on. Advanced reading and writing instruction ranked 19th. Teaching academic self-confidence, leadership skills and social emotional learning all ranked higher than teaching above grade level content.

The relatively low incidence of providing gifted pupils with accelerated math and ELA reveals a great disservice to gifted pupils. Withholding advanced math/ELA curricula may be aimed at slowing the measurable academic growth of the top pupils, as part of a plan to close achievement gaps, close excellence gaps, and achieve "equal outcomes."

More about the needs of gifted pupils in this old post.
More about acceleration (full grade acceleration and subject acceleration) in this old post.
More about grading practices which hold back gifted pupils and help create "equal outcomes" in this old post.

Other NCRGE Key Findings reveal additional areas for improvement in gifted education.