He sits in the witness chair, taut as a bowstring. He speaks slowly but intensely, his slender frame bent forward as he peers through thick glasses. His voice conveys an odd mix of defiance and resignation.

He is Gregory Gibson, 33, sometime police informant, convicted criminal and now maverick witness in a major civil rights trial in federal court here.

In the trial, three former and present Prince George's County police officers, including the second-highest-ranking official in the county police department today, are accused of staging a series of convenience store holdups in 1967 at which officers lay in ambush, killed two men, wounded a third and arrested others.

Gibson is a key witness against the officers. Much of the case may stand or fall on his testimony.

The accused officers, members of a "death squad," as it was called in police circles, are being sued for $9 million by two of the arrested persons and families of the two slain men for alleged deprivation of their civil rights.

For six days over the last two weeks, Gibson testified against the officers--a reluctant, hostile and unpredictable witness leading judge and jury through a morass of half-answers and equivocations, of admitted lies within truths and proclaimed truths within lies.

His six days on the stand provided a glimpse, as seen through the prism of his often bitter words, into the life of the petty thief-become-police informant, the hustler trading information for dismissal of a criminal case or a reduced prison sentence in an intricate web of cooperation and obligation with police.

"I felt I was a person that was just being taken advantage of," he testified. " . . . You're already convicted, and the best thing you could do was pray that somebody would give you a break."

At one point, he said he had told the plaintiffs in the trial he would say "anything they wanted" for $50,000, and would "disappear" for the police for a similar amount. He said he received no such sums.

He also said his life had been threatened, hinting at reprisals by criminals he may have implicated in testimony.

Buffeted by cross-examining police lawyers, he branded some court documents as fabrications, said many of his own statements were "lies" and once nearly threw the case into a mistrial when he announced he would no longer continue testifying.

He agreed to resume testifying, however, after presiding Judge Herbert F. Murray ordered in deputy marshals to keep him from leaving the courtroom and summoned a federal Public Defender attorney to advise him of the consequences--jail time--of being in contempt of court. Gibson completed his testimony last Thursday.

Born in Boston, Gibson moved to Maryland and grew up in the burgeoning black suburbs of Prince George's County in the 1950s and 1960s.

In school, he was in "bits of trouble, fighting, that sort of thing," he said. His mother pulled him out of Fairmont High School and sent him to the Maryland Training School for Boys as "ungovernable."

Later, he worked at odd jobs in Seat Pleasant and College Park, he said, until he was arrested in early 1967 for a 7-Eleven holdup. That was when he first met Prince George's County Police Det. Joseph D. Vasco Jr. and became an informant for him in exchange for possible leniency in criminal cases against Gibson.

Later that year, Gibson testified, Vasco, who is now No. 2 man in the police department, directed him to recruit two men to rob a Highs store in Adelphi where a team of stakeout officers would be waiting.

Gibson, then 18 years old, said he agreed to try. "I was afraid . . . .If I said no, he was a police officer and could tell the judge handling an unrelated case against Gibson and it would make it even worse."

Gibson said he succeeded in recruiting two men, William Hunter Matthews and Marvin Rozier, both 18, and went with them to the store on June 8, 1967. In the course of the holdup, Matthews was shot and killed by police shotgun fire and Rozier was arrested.

Police have challenged Gibson's version, claiming Gibson reported to them that the robbery was already planned and they then staked out the store.

Matthews, they said, was armed with a .32-caliber pistol and was shot when he ignored a police warning to halt and fired at an officer.

Gibson's account was further challenged when defense attorneys introduced tape recorded interviews by both county and state police investigators in which Gibson appeared to contradict his trial testimony, acknowledging that Rozier and Matthews were "ready" to commit the holdup without being recruited or persuaded by Gibson.

Gibson, glaring defiantly at defense attorneys and speaking with bitter resignation, declared in his testimony that much of what he told the investigators were "lies."

"I told them what I believed I should say . . . to keep my life safe," he testified at one point. At another, he said he was willing to say anything to get investigators "to leave me alone."

Last Thursday, after his dramatic announcement that he would no longer testify, he told Judge Murray, "Everyone's trying to sway me, to have me say what they want me to say."

Today, Gibson lives in Baltimore, trying to rebuild his life, he says. He is married and works as a spray painter. He says he wants to be left alone.

"I'm just an ordinary person . . . trying to work for a living and trying to maintain my home," he told Judge Murray last Thursday. " . . . I got a late start in life."

The trial, which started last November, is expected to last another six weeks.