Gasoline prices could rise by 6 cents a gallon over the next few months if the Organizatoin of Petroleum Exporting Countries calls for a modest increase in oil prices next month and if President Carter ends gasoline price controls next January as planned, according to Energy Department officials.

More than half of the likely increase will come from administration plans to end gasoline price controls, not actions by the oil cartel.

According to an environmental impact statement drafted DOE and released this week, decontrolling gasoline would raise prices from 3 to 4 cents a gallon. This finding, however, is at odds with repeated assurances by Energy Secretary James R. Schlesinger Jr. that ending gasoline price controls would have little, if anY, effect on fuel prices.

"It is possible that gaoline prices would go up 3.7 cents more than if regulations continued," says Douglas Robinson one of DOE's top regulatory officials.

As for the remaining price increases expected next from OPEC, an oil price increase of 10 percent or less on the current $1270 price of a barrel of crude oil would result in about 2 cents more per gallon in gasoline prices. In recent weeks senior State and Energy Department officials and international oil industry executives have been saying that an OPEC price rise in the rang of 5 to 10 percent is expected to result from the cartel's December price-fixing meeting in Abu Dhabi.

Robinson, deputy administor of DOE's Economic Regulatory Administration, said that gasoline decontrol could also widen the current 4-cent-a-gallon spread between leaded gasoline and higher-Priced unleaded gasoline. Under gasoline decontrol, however, Robinson said "we do not expect the differential to go up to 7 cents a gallon," as suggested in the environmental impact statement.

Currently half of oil products sold in the United States are under pricing ceilings, with gasoline making up nearly all of that. Controls on gasoline have been in place since 1973.

President Carter unveiled plans to decontrol gasoline when he sent his natinal energy plan to Congress in April 1977. Because of concern about the political impact of gasoline decontrol on efforts to win support for the energy plan, Schlesinger and other administration officials held back sending the required decontrol plan to Congress.

Decontrol can be blocked if either the Senate or House vetoes it 15 days after it is sent to Congress.

Last August Rep. John Moss (D.Calif.) charged that DOE's plans for decontrol would raise prices unnecessarily and could precipitate a shortage of gasoline within the next three years.

Oil industry executives such as Jack Blum of the Independent Gasoline Marketers Council counter this, saying that the DOE's pricing regulations are outmoded, uneconomic, and anti-conpetitive. They also say that by lifting contorls there will be greater incentives to make adjustments to refining capacity in order to produce more unleaded gasoline.

Clarence Ditlow of the Center for Auto Safety, a consumer group operating under Ralph Nader's umbrella, disagrees, Ditlow says that price decontrol would raise unleaded prices by as much as 8 to 10 cent a gallon. The result would be that motorists would switch to lower-costleaded gasoline, adding to air pollution.

In a related action, DOE has issued a ruling that would allow gasoline stations to edge up their prices in December by about 2 cents a gallon to account for rent and wage increases. The increase expected under decontrol.

Over the last weeks, the market for unleaded gasline has tightened up a bit, especially for some Shell Oil Co. Stations. Overall, however, DOE officials rule out a serious gasoline shortage this year or next.