The private gambling firm that operates the D.C. instant-winner lottery game has paid its general manager more than $90,000 for her work over the past year and given her use of a $21,000 leased car and a $203,000 furnished town house in Southwest Washington, according to company officials.

Games Production Inc. (GPI), which has earned more than $4 million since winning the lottery contract last summer, also spent more than $37,000 last year to house general manager Gloria A. Decker and the firm's temporary offices in a local hotel.

GPI officials and the chairman of the D.C. lottery board yesterday defended the firm's outlays to Decker and other expenses as legitimate costs for a multimillion-dollar business.

"I don't think how the company spends its profits has anything to do with the contract," Decker, a former executive director of the New Jersey Lottery Commission, said in an interview yesterday.

Questions about the firm's operations--particularly about $2.2 million paid to La Mancha, the advertising arm of GPI--have been raised over the past several days by Scientific Games, a former partner company that has accused GPI of mismanagement and financial irregularities.

Decker dismissed the allegations made by Scientific Games as an effort to discredit her company because the two firms are likely to bid against each other to operate the city's lottery in fiscal year 1984, which begins Oct. 1.

Mayor Marion Barry and City Council member Nadine P. Winter (D-Ward 6), who oversees the lottery board, have cited GPI's costs and profits as one reason the city should consider running the instant game itself.

The District--the only jurisdiction in the nation that contracts with a private firm to run its instant lottery--receives proportionately less revenue on its game than all but two others in the country. The lottery board yesterday voted to commission a study by a recognized accounting firm on whether the city should run the operation itself.

Decker alleged yesterday that Scientific Games had raised questions about GPI's operations in order to embarrass the lottery board as it prepares to award new contracts.

"We're the ones who have to keep defending ourselves because of all the money that's coming in," said Decker.

She suggested that Scientific Games, an Atlanta-based gambling firm, should be required to explain how it has spent the several million dollars it has earned.

Decker, who earned $55,000 in her New Jersey post, joined GPI last May. A Democrat, Decker was succeeded in New Jersey by Hazel Gluck, a Republican. Gluck said yesterday that Decker "had the highest integrity."

Decker declined to discuss in detail the operation of GPI. She said the firm is preparing to bid on the city's new contracts, and would not reveal how much of the more than $4 million it has earned has actually been spent on lottery operations.

But in response to published reports about the firm's expenses, Decker defended spending approximately $70,000 for rent, security and insurance at three ticket distribution centers that also are check-cashing businesses run by the family of William N. Suter, the chief executive officer of GPI.

Decker also said she is not troubled by the makeup of the ownership of La Mancha, the advertising firm--which includes Teresa Suter, William Suter's wife--because there are also other owners.

Games Production initially was paid 7.8 percent of gross ticket sales, but earlier this year cut that amount to 6 percent to increase prize money to stimulate sales. The lottery has made about $15 million more than expected this year, generating an extra $1 million for the firm.

Instead of the original estimate by the board that 40 million of the $1 lottery tickets would be sold this fiscal year, bettors have already purchased more than 55 million tickets with three months to go.

The board said yesterday that any future instant lottery operator would receive no more than 4 percent of gross revenues to operate games that begin after Oct. 1.

In addition, the board recently promised the House Appropriations subcommittee on the District, which oversees the city's purse strings, that it would reduce its overall costs in operating the game from about 25 percent to between 11 and 14 percent next year.

Lottery board Chairman Brant Coopersmith attributed the high costs to the first year of operation and the need to ensure that the lottery did not fail because of mismanagement.

"The worst thing that could have happened is that the lottery had not gotten off the ground," Coopersmith said.

He said it was better for city officials to be fighting over a successful lottery than one that had failed. CAPTION: Picture, GLORIA A. DECKER . . . has use of a car and a $203,000 house