What's the difference between ETL and UL listed electrical outlets? Which should I purchase?
It appears California leaves recognition of testing labs all the way down to city level, an example of this is the City of Los Angeles, but there should be no issue with ETL for GFCI receptacles.
The National Fire Protection Association (also a private company and owner of the NEC) doesn't specify OSHA NRTL's or name UL or any other testing labs in NFPA70 (the NEC). It says:

110.3(C) Listing. Product testing, evaluation, and listing (product certification) shall be performed by recognized qualified electrical
testing laboratories and shall be in accordance with applicable
product standards recognized as achieving equivalent and effective
safety for equipment installed to comply with this Code.
Informational Note: The Occupational Safety and Health Administration
(OSHA) recognizes qualified electrical testing laboratories that
perform evaluations, testing, and certification of certain products to
ensure that they meet the requirements of both the construction and
general industry OSHA electrical standards. If the listing (product
certification) is done under a qualified electrical testing laboratory
program, this listing mark signifies that the tested and certified
product complies with the requirements of one or more appropriate
product safety test standards.

Many Authorities Having Jurisdiction defer recognition by ordinance to labs recognized by OSHA as NRTL's. Not all NRTL's do electrical testing, and not all approvals apply to all countries. For example a UL symbol with the letters AR below it are only approved for use in Argentina, and NSF doesn't test electrical.