The Office of Management and Budget plans to forecast today that Congress is on target to satisfy the $110-billion Gramm-Rudman law deficit target for the coming fiscal year, congressional sources said. The officials, who asked to not be identified, said that by the government’s updated calculations, the deficit for the 1990 fiscal year--which begins Oct. 1--would be about $99.5 billion. Even if $5.7 billion in income from planned sales of federal assets is not included--and it cannot be counted under Gramm-Rudman procedures--the deficit would be $105.2 billion, still well within the law’s limit. Because of disagreements between the Administration and Congress, few people expect the assets to be sold. If the government projects that the $110-billion shortfall target will be surpassed, Gramm-Rudman automatically triggers spending cuts.