undecided or major

Is it easier to apply undecided to UCI than a major if you had a 3.6 gpa? How many people do get admitted with a 3.6 gpa as undecided to UCI?

Going in as Undecided can be problematic sometimes depending upon the competitiveness of the major you ultimately plan to change into later.
UCI admits into the University first and then into the major, so you should specify some major as your first choice and you can select Undecided as an alternate.

You can google the requirements to declare a specific major at UCI, but going in undecided will not give you a boost in admissions.

Freshman admit rates for UC GPA of 3.40-3.79 (capped weighted) and not major specific:
UCI: 11.1%