CG 22 68 is going to provide liability coverage for the insured for the use or maintenance of a customer's auto. This is liability coverage in case a third party is injured or damaged through the use of the customer's vehicle. It does not provide coverage for any damage done to the customer's vehicle by the insured since the CGL form contains an exclusion for property damage to personal property in the care, custody or control of the insured, and the endorsement does not address this exclusion.

GKLL coverage provides coverage for the insured if he damages the customer's auto while servicing it. This is based on the insured being legally liable for the damage. Of course, the insured can buy direct primary coverage so that the coverage is applicable without regard to the insured's legal liability for a loss to the customer's car.

So, if the insured is just interested in having liability coverage in case he injures someone while servicing or repairing a customer's car, then CG 22 68 is fine. But if the insured wants coverage for any damages he causes to (or is responsible for) the customer's car while it is in his care, custody, or control, GKLL coverage is needed.