FOR MORE INSIGHT into the roots of the problems at the General Service Administration, consider the case of Gateway Center, a new office building in Philadelphia that GSA leased for $52 million in 1970. The project was sponsored mainly by a former law partner of former Senate minority leader Hugh Scott (R-Pa.). The lease, endorsed by Sen. Scott and other Pennsylvania Republican leaders, was worked out by former GSA administrator Robert L. Kunzig, who had been Sen. Scott's administrative assistant, and former public buildings chief Arthur F. Sampson, whom Mr. Kunzig had brought into GSA.

All those "formers" were current at the same time - and their connections were very friendly. Indeed, according to FBI reports, Mr. Kunzig started talking with Gateway's backer five months before GSA publicly made known its need for more Philadelphia office space. Besides helping to promote the project, Mr. Kunzig and Mr. Sampson approved the lease even though the project's backers had not met GSA's usual requirements of firm financing, a building permit and the like - and even though other developers had offered space for less.

You might think there is something illegal about that kind of deal. The FBI did dig into it, but the Justice Department decided against prosecution just before the five-year statute of limitations ran out a few years ago. The reasons for non-prosecution are interesting, too. The FBI found that Gateway's developers had trumped up some of the documents involved - but a charge of misleading GSA didn't stand up, prosecutors decided, because GSA officials had not been misled; they knew just what was going on. That may suggest a possible conspiracy to defraud the government. But such charges are very hard to prove without insiders' testimony or other evidence of some payoff, kickback or other benefit to the officials involved. Nothing of that sort was found.

An old Tammany Hall leader, George Washington Plunkett, had a fine name for insider deals that didn't involve payoffs or other honest acts. His term was "honest graf," which he summed up as: "I seen my opportunities and took 'em." That is the spirit in which GSA apparently was run for many years. And it's simple to see how such conduct at the top of GSA could foster in the ranks an atmosphere that encourages and condones demonstrably dis honest graft - such as the payoffs of supply-store managers alledged in the indictments handed up last week.

That, we think, well defines the challenge facing President Carter and GSA Administrator Jay Solomon now. Besides prosecuting those who have broken laws, they will have to change the whole climate in which the agency's business is done.