From the first days of the Reagan administration, back when "gypsy moths" were nothing more than grown-up caterpillars and "reconciliation" had to do with emotions, not outlays, David B. Gerson's name was appearing in government directories right under Budget Director David A. Stockman's.

Gerson's title is different now -- instead of being executive assistant to the director of the Office of Management and Budget, he is an associate director -- and his powers are broader. But the essence of his job is what it was four years ago. If Stockman is the architect of the federal budget, Gerson is his general contractor.

"The best way to describe the job is to think of an hourglass," he says. "All things going to the White House go to my office first. All things coming in to OMB from presidential assistant Craig L. Fuller go to me. They staff a speech draft to me for checking, and I staff it out here."

Several high-ranking OMB officials, career and political, give him high marks for his general understanding of the fiscal and political issues from which the budget is spun, but that is not his forte.

What he does, and, by all accounts, does well, is distill ideas, condense documents, dispatch paper and people to appropriate places at appropriate times, rework computer programs, recruit personnel for budget examiner's jobs, return phone calls from Capitol Hill and keep more than 100 carefully itemized briefing books up to date, all with a keen sense of order, purpose and certainty.

In fact, an aura of certainty permeates everything he does. He remembers fine details of past campaigns and budget battles, political war stories that seem incongruous when told by a man who still looks like an earnest high school student council member.

One matter on which Gerson receives friendly and unfriendly criticism -- all of it anonymous -- is his style in handling subordinates. "If he has one fault that needs improving it's his sensitivity," one high OMB official said.

Gerson argues that this is unfair and points to his efforts to promote career development for middle-ranking OMB employes. High turnover on his immediate clerical staff, he said, happens because "our office is a very difficult place to work because we work very hard."

"Ger," as he is known to all at OMB, is one of three aides who came down Pennsylvania Avenue from Stockman's congressional office in 1981. The others, associate directors David W. Moran and Frederick N. Khedouri, are more directly involved in policy formation. All three, like their boss, are gluttons for work.

"Moran is the ideological avant-garde. Khedouri is the can-do person," said one ranking OMB official. "Gerson" -- the official pauses a moment -- "I don't like the word facilitator, but that says part of it."

"As Dave Stockman's audience has grown over time, Gerson is always the keeper of the body," said Moran. "It's important for Stockman to be counseled to do the things he should do and not spend time on what he shouldn't waste time on. . . . You can make all the policy you want to, but if you don't have the machinery to make sure things get where they want to go, you might as well be sitting in your bedroom talking to the mirror."

Gerson's view of his job is exemplified by a meat cleaver displayed on his bookshelf. Whirlpool Corp. -- a Michigan constituent -- gave identical cleavers to all three aides when they moved into the Old Executive Office Building. It is inscribed: "May this be the symbol of your new mission."

As if punctuating his certainty about what should be done and how, the pronoun "I" figures prominently in most of his sentences. It does not come across as braggadocio. He is merely sure of his place in Stockman's history and his own.

"I'm trying to make sure that the resources of the agency keep one-half step ahead of the things Dave Stockman has to do," he said. "That means carrying a lot of pieces of paper in a lot of different directions . . . . "

The centerpiece of Gerson's job is Stockman. When his peers at Case Western Reserve University were majoring in government or literature or business, Gerson was majoring in Stockman.

In 1973, as a congressional intern, Gerson, then 20, helped Stockman revamp the operation of the House Republican Conference, of which Stockman was executive director. After two summers here, Gerson could not leave. Instead of going back to school, he stayed on with Stockman.

But he goes on at length about the ensuing years: moving to Stockman's parents' home in Michigan to run Stockman's first campaign in 1976, shopping for a computer for the congressional office only days after winning the Republican primary that year, becoming an administrative assistant at the age of 22, one of the whizziest kids in a whiz-kid world.

He seems proudest of his work in getting Stockman's professional life computerized, from the days when the 1976 congressional campaign bought time on a bank computer in Benton Harbor, Mich., to the Capitol Hill years when he computerized everything from constituent correspondence to the tracking of federal grants in the district.

When Stockman sought to turn OMB's computer system into an instant-answer device that could do everything from comparing programmatic spending levels under different administrations to figuring the cost of legislative proposals in a few hours' time, Gerson was dispatched to help organize it. Then, Moran said, "Gerson extended it into something universally available at OMB."

What happens when Stockman leaves OMB? "At some point I do see the need to separate myself from the notion of being in Dave's shadow," Gerson said. "But the people who work closely with us do have a firm notion of my work as separate from Dave's . . . . I haven't sought another job."