New York City's goal of a balanced 1982 budget without federal assistance is being made more difficult by City Hall's use of gimmicks and fictions in its budget projections, a Citizens Budget Commission report charged yesterday.

Mayor Edward Koch's successful plea to Congress for new federal loan guarantees this year was made on the basis of a four-year plan that would bring restored fiscal health and a fully balanced budget in 1982.

The Congress approved $1.65 billion in guarantees and President Carter is planning to sign the bill at a ceremony outside City Hall Tuesday.

The civic watchdog organization's report charged that the Koch administration has blurred the cost of its recent labor settlement with the municipal unions by gimmickry and has relied on its budget planning on unapproved federal funds that may not be forthcoming.

The impact of the recent labor agreements will be severe in the 1981 and 1982 budgets, the report claimed, when an additional $500 million will be needed each year.

The report said that the official estimate of the labor settlement, $749 million, is inaccurate and that the true cost is "well over a billion dollars."

The city's plans to cover budget gaps through 1982 also are based on a number of unspecified city economies, the report said. Without these and without the anticipated federal and state funds, the city's 1982 budget gap would be from $684 million to $1.3 billion, the report said.

Such gaps would force the city to seek new federal aid to avoid bankruptcy.

Even if the federal government agrees to assume the city's welfare and Medicaid burdens and if city tax revenues increase while larger state and federal funds become available, the city's budget gaps over the next four years will be greater than projected by the Koch administration's planners, the report said.

It urged the city to take immediate steps to cut expenditures "while there is still time to effect expenditure cutbacks gradually and intelligently."

The Citizens Budget Commission was founded in 1932 by civic and business leaders to monitor New York City's finances and operations.