Most majors at most UC campuses accept IGETC in substitution for their general education requirements, but not all do. CSUs should accept IGETC in substitution for their general education requirements, since they are closely aligned with CSU general education requirements. But note that there are slight differences in IGETC for UC and CSU (foreign language for UC, oral communication for CSU).
Completing IGETC or general education is not necessarily required for transfer admission (check each campus and major); if you are admitted without it completed, you need to complete the remaining general education requirements specified by the campus before graduation.
http://www.assist.org can be very helpful to you, assuming you are a California community college student aiming to transfer to a UC or CSU.