Prices for all grades of gasoline dropped during the past month in Northern and Central California and in Nevada, the California State Automobile Assn. reported. The auto club said it was the first time since August, 1986, that all gas prices have dropped in one month, but unleaded and leaded fuels were up 13 and 15 cents over December, 1986, prices at self-service stations. In a survey of more than 600 retailers, the auto club found unleaded regular self-service gasoline down 0.8 cents and selling at 98 cents a gallon. Leaded regular self-service gasoline was 92 cents, down a penny.