By MURRAY WEISS and MARSHA KRANES

An iron-nerved FBI agent infiltrated the Gambino crime family and uncovered a raft of rackets that included extorting wisecracking radio DJ "Goumba Johnny" to get free ad time for mobbed-up nightclubs, authorities charged yesterday.

Manhattan U.S. Attorney David Kelley said the Gambinos "forced a radio station to run advertisements and conduct promotional events involving Gambino family businesses, including those whose profits the Gambinos were extorting."

Neither the station nor its popular morning DJ were identified in the sweeping 53-count indictment.

But law-enforcement sources told The Post that it is 103.5 WKTU, home station of John "Goumba Johnnie" Sialiano.

The DJ is a former buddy of Gregory DePalma, a top Gambino capo who was among those charged yesterday in the $30 million gambling, fraud and extortion operation.

Sialiano served a short jail stint on tax charges stemming from the notorious Scores racketeering case of the 1990s.

The DJ, who was once a bouncer at Scores, was not the only extortion victim at WKTU, the sources said — his radio sidekick, Johnny Rocket, was also squeezed for ad time.

The Gambinos forced the pair to give freebie plugs to clubs in Manhattan and The Bronx where the family either had a financial stake or was skimming the till, the sources said.

One of the businesses to which the pair gave free on-air publicity was a Bronx strip joint, the Mirage International Cabaret. The undercover agent had invested in the club to infiltrate the mob and get close to DePalma, the sources said.

Sialiano could not be reached for comment yesterday, and a spokeswoman for Clear Channel, which owns WKTU, said station executives were not available.

In all, 32 men were arrested and charged with using beatings and death threats to extort money from restaurants, nightclubs and trucking and construction companies, according to the indictment.

In carrying out its probe, the FBI even bugged the nursing-home room of mob capo DePalma's comatose son.

Kelley said the elder DePalma, his crew and mob associates "engaged in daily meetings and conversations about their ongoing criminal enterprise in the room in the mistaken belief that they could . . . avoid law-enforcement scrutiny."

"They went into the nursing home, and we were right behind them," he said.

In another bizarre, out-of-a-mob-movie detail from the indictment, two Gambino family members — one of them DePalma himself — allegedly found a creative way to deal with a "recalcitrant" loan-sharking customer who had fallen behind in his payments.

They beat the victim with a candlestick holder in the housewares department of Bloomingdale's in White Plains.

Kelley said the Gambinos netted $20 million from illegal gambling and $10 million from fraud and extortion while the sting was carried out.

Pasquale D'Amuro, head of the FBI's New York office, credited the undercover agent with blending so seamlessly into the underworld that he was able to make hundreds of secret recordings revealing the family's inner workings.

It was a feat, officials said, that had not been seen since FBI agent Joe Pistone's work in the Bonanno family a quarter century ago — under the alias of Donnie Brasco.

Pistone's portrayal brought down the leadership of the Bonanno crime family.

This time around, the FBI's mole was so convincing that he got himself listed as a member of DePalma's crew — and DePalma recently discussed sponsoring him as a made member of the mob, Kelley said.

The arrests included Gambino family acting boss Arnold "Zeke" Squitieri of Englewood Cliffs, N.J., underboss Anthony "Machiavelli" Megale of Stamford, Conn., and the now-jailed capo DePalma, who was busted and sent up the river with John "Junior" Gotti in the Scores case.

Squitieri had allegedly taken over as acting Gambino boss after Peter Gotti was convicted in two separate trials last year.

Squitieri was awaiting arraignment in federal court in Manhattan; a call to his attorney was not immediately returned. Megale, already in custody on a separate racketeering indictment in Connecticut, denies he's underboss, said defense attorney Stephan S. Seegar. Both defendants face up to 20 years in prison if convicted.

"This is a case where this hierarchy was in place for a very short period of time before we swooped in and were able to get them out," Kelley noted.

Most of the others busted yesterday were identified in the indictment as Gambino family members or associates.

Their illegal activities, according to the indictment, included:

* Embezzling money from a union pension fund — and using the proceeds to buy health insurance for themselves and their families.

* Running an illegal gambling ring and Super Bowl betting pools.

* Trafficking in counterfeit goods and stolen property — including cars, watches and other jewelry.

* Loan sharking.

In one recent incident cited by Kelley, an indebted victim was forced to pick up the tab for a Las Vegas vacation for mob members and their wives.

The FBI also wiretapped DePalma's cellphone," which gave us an insider's view of the DePalma crew and the Gambino family hierarchy," Kelley said.

He praised the undercover agent for "risking his life for two years" acting as an underworld mole.

"As long as there are greedy gangsters seeking to make a fast buck, the FBI will be there — and sometimes, we will be watching and listening from very close range," D'Amuro said.