“I go down to Apple and I’m wearing a suit — I’m sure they thought I was wearing a costume — and then the Grim Reaper escorts me to the conference room,” he said.

Mr. Reback helped develop the practice of representing what antitrust lawyers call “third-party complainants.” Put simply, companies pay him to complain to the government about their larger competitors in hopes that it will provoke the antitrust authorities to do things like scuttle merger proposals or sue for abusive behavior.

“If there were intellectual property rights for creating this niche, everyone would be paying Gary royalties,” said William E. Kovacic, a law professor at George Washington University.

If this sounds like a suspicious job description then it will be easy to understand why it is hard. The main hurdle is that when a lawyer like Mr. Reback comes complaining, regulators are inclined to believe the clients are whiners who cannot compete. Convincing them otherwise requires detailed economic analyses, which is where Mr. Reback’s claim to legal fame comes from.

In the 1990s, he and a colleague, Susan Creighton — a Wilson Sonsini partner who now represents Google — wrote a white paper that advanced a novel economic theory on “network effects.” The paper was one of a number of complaints about Microsoft, but it has gone down in legal lore for pushing the Justice Department to start investigating the software giant for antitrust violations.

The central idea in the paper was that technology, more than any other industry, tends to favor the largest competitor because the “network” is itself a competitive advantage. That is, users have to use the same thing everyone else is using. What this means, Mr. Reback said, is that in tech, antitrust law must be enforced more swiftly.

“The more you have, the more you get,” Mr. Reback explained.

A more recent project, produced in 2013, was a 62-page analysis of Google’s behavior in Europe. It argues that the company is deliberately trying to push comparison-shopping sites lower in its search results, not so much because it wants to take business from them, but because it doesn’t want comparison-shopping prices to show up near product ads. If that happened, consumers might be reluctant to click on those ads, he argued, and clicks are how Google is paid.