Donna Jacobs sputtered with rage yesterday as she watched an attendant at Rock Creek Gulf on Adams Mill Road NW pumping unleaded gasoline into her Celica at $1.20 a gallon.

"It's ridiculous, it's highway robbery, but . . . I'm afraid I can't get any tomorrow," said Jacobs, a Navy lieutenant j.g. planning to drive to Charleston, W. Va., early today to spend Christmas with her parents.

But if an analysis by Energy Secretary Charles Duncan is right, that same gasoline could be costing Jacobs $1.28 a gallon in a few weeks in the wake of the lastest round of OPEC price increases.

And it could cost $1.50 a gallon by the time Jacobs and other Washington area motorists are ready to take their vacations in July, according to at least one oil industry analyst.

The Lundberg Letter, a respected gasoline marketing weekly, found in a recent worldwide survey that regular is selling for more than $2 a gallon in 12 European Countries and nearly $3 a gallon in Italy and Switzerland.

The average price of gasoline in Washington was already more than $1.05 a gallon before Duncan's annoucement Thursday that the new round of world oil price increases would add 4 to 8 cents a gallon at the pump in coming weeks.

That could bring the average price here to more than $1.13 a gallon by the new year -- or soon thereafter.

Duncan also said that home heating oil would go up 3 to 17 cents a gallon. The average price for that product is already nearly 91 cents a gallon for the more than 100,000 homes that use it here, according to a Washington Post survey yesterday of 15 local oil distributors.

If Duncan is right, that could make the average gallon of home heating oil cost 98 cents here -- plus tax in the District of Columbia and Virginia. oMaryland no longer taxes heating oil.

The average price of gasoline here has increased at least 33 cents a gallon -- about 50 percent during the last year, according to statistics supplied by the American Automobile Association and the oil companies.

The price of home heating oil has increased somewhere between 50 and 100 percent during the same period.

At the same time, the public has become accustomed to tight supplies. While there is about 7 percent less gasoline available for sale this month than last December, there are few lines at gasoline stations -- and those are short ones.

"The motoring public has made an outstanding adjustment," said Glenn Lashley, an AAA spokesman. "People aren't using all the gasoline that's available."

Lashley and others estimate that more than half of the decreased demand for gasoline in the United States is caused by the soaring prices. The inconvenience of facing gasoline lines, and other factors, account for the rest of the reduction in demand.

"People are grumbling about the prices," said Alexandria Exxon dealer Don Youngman. "They're saying 'I guess Carter's pushing for $2 gallon.'"

Youngman was charging more than $1.02 for regular yesterday, having just received a 6-cent-a-gallon increase on all grades of gasoline from his supplier, Exxon.

Exxon raised its prices dramatically this week as a result of the world oil price increases.

Lundberg Letter editor Mark Emond said gasoline prices will rise "appreciably faster" in 1980 than in 1979. In the last year, he said, the national average price went up 38 cents a gallon.