A former Prince George's County police informant claimed in court here today that a tape-recorded interview of him by state troopers investigating a so-called police "death squad" was fabricated. An hour later, however, the ex-informant, Gregory Gibson, acknowledged the tape was genuine.

The reversal was the latest of several surprise actions by Gibson since he began testifying last week in the $9 million civil trial in federal court against three alleged "death squad" members accused of staging a series of robberies in 1967 in Prince George's County, in which two men were shot and killed by police and two other men arrested.

Last Thursday, Gibson told presiding Judge Herbert F. Murray he feared his "life might be taken" if he continued to testify this week. He gave no specific evidence of any danger and returned to court this week unharmed.

Then on Monday, he acknowledged signing an affidavit that defense lawyers said contradicts his claim that police asked him to recruit participants to commit robberies. The affidavit has not been introduced into evidence.

Today, Gibson initially branded as a "forgery" a confession bearing his signature in which he admitted committing a 7-Eleven store robbery in early 1967 and implicated an accomplice.

Attorneys for the police are now seeking to discredit Gibson, portraying him through cross-examination as evasive, vacillating and inconsistent about details of the robberies and his informant relationship with police.

Originally, Gibson, 33, testified that police Det. Joseph D. Vasco Jr., now the second-highest-ranking official in the county police department, directed him to recruit participants for a robbery of a High's dairy store in Adelphi where police would be waiting.

In the robbery, which occurred June 8, 1967, William Hunter Matthews, 18, was shot and killed by police staked out at the store, and a second participant, Marvin Rozier, 18, was arrested. Gibson, who had accompanied Matthews and Rozier, was allowed to "escape," according to attorneys opposing police.

Allegations of the "death squad" stakeout tactics were first publicly disclosed in a series of Washington Post articles in February 1979, almost 12 years after the High's robbery and another similar incident in November 1967 when a second man was shot and killed by police.

Both Prince George's County and Maryland State Police investigators began interviewing persons named in the Post articles. It was the state police tape-recorded interview of Gibson, conducted in July 1979, which Gibson at first said today was fabricated.

His claim came as defense attorneys were trying to get Gibson to acknowledge that a portion of the tape, in which he said he told Vasco two would-be robbery participants were "doing stick-ups" in the Adelphi area, contradicted his testimony in the current trial that he had not given Vasco such information.

Out of the presence of the jury, Gibson, a slender, intense man wearing thick glasses, said, "That's not me on that tape."

"You're saying the tape is a complete fabrication?" asked presiding Judge Murray.

"Yes sir," said Gibson. " . . . It sounds like myself in certain tones, but they screwed up the language."

Attorneys in the case huddled with Murray in a whispered bench conference. Murray then turned to Gibson, told him the attorneys all agreed the tape was "genuine" and asked him to listen to the entire 40-minute tape to determine if he, too, thought it was genuine.

Gibson agreed. An hour later, after listening to the tape in a private room with a lawyer from each side of the case present, he said the voice on the tape was in fact his.