What is the "gatekeeper" model used by managed care plans?

The HMO's primary care physicians would determine the drug formulary that was to be used by the HMO's patients.

An HMO's "gatekeeper" would determine whether an HMO enrollee would have access to experimental treatments to be paid for by the HMO.

An HMO's "gatekeeper" would determine whether a patient could be referred to specialists and$/$or hospitals unaffiliated with the HMO.

An HMO enrollee would choose a primary care physician, who would then control the enrollee's diagnostic and specialist referrals.