In 2021 I helped my mom sign up for the Medigap Plan G (High Deductible). She had relocated from the East Coast. We went with Mutual of Omaha. For all these policies there are only a few of companies to go through for California, and not all offer each plan. I think there is a plan-finder on Medicare.gov that will tell you exactly which companies will sell a particular plan. Eventually, you'll just have to get on the phone with one company or the other to verify the cost and sign up. [99% of info on the web is geared toward having you sign up for Advantage plans because that is where the money is for insurance companies.] Good luck.