There is no simple numerical cutoff, except that the UCs have a systemwide minimum UC GPA of least 3.00. Note that this is the GPA as calculated by UC, which is different from the way your high school does it.
The UC system is supposed to find a place for anyone who is in the top 9% of their high school class (“eligibility in the local context”). But different high schools have different grading standards, so the same GPA could be UC-eligible at High School A and not at High School B.
Alternatively, you are UC-eligible if you rank in the top 9% of a statewide scale that also considers standardized test scores (“eligibility in the statewide context”). But in that case, the same GPA might or might not be UC-eligible, depending on the associated SAT or ACT score.
The typical UC GPA range for admitted freshmen at UCSC for Fall 2017 was 3.70 - 4.13. Half the admitted students (50%) were in that range. Another 25% were lower than 3.70, and another 25% were higher than 4.13.
http://admission.universityofcalifornia.edu/campuses/santa-cruz/freshman-profile/
However, many of the best-qualified students who are admitted to UCSC are only using it as a safety, and don’t actually go even if accepted. Most of the students who actually end up enrolling at UCSC have high school GPAs in the 3.25 - 3.74 range, as per Section C11 here:
https://mediafiles.ucsc.edu/iraps/common-data-set/common-data-set-2017-18-revised.pdf