Joyce Gowie Gamble, the founder and president of a Washington investment-management company, was convicted yesterday of defrauding 56 customers out of $250,000.

Investors in National Investment & Retirement Accounts Inc. were promised a 15 percent or higher return on their money in 1984 and 1985, at a time when banks were paying 11 to 12 percent. Documents assured clients that the funds were being invested in accounts receivable, limited partnerships, mortgage securities and business loans. In reality, almost all the money was used by Gamble to run NIRA and to buy a house in Virginia that was to be her residence, according to Assistant U.S. Attorney Rhonda Fields.

After four hours of deliberation, a U.S. District Court jury of six men and six women in Washington found Gamble, 37, guilty on all 11 counts of mail fraud and interstate transportation of property taken by fraud. Sentencing was set for May 1. The maximum penalty for each mail fraud is five years in prison and a $1,000 fine. The other crime carries a maximum penalty of 10 years in prison and a $10,000 fine.

The prosecution claimed that Gamble tried to buy a $272,000 house in Virginia. The deed was to be in her name, but the mortgage payments were to be made by NIRA, while Gamble was to pay rent to NIRA. The sale was canceled when financing fell through. The rest of the $250,000 went for a car leased for Gamble, miscellaneous expenses, and salaries and fees for employes, consultants and lawyers.

Defense attorney John Mercer argued that the defendant showed good faith, if not good business judgment, and that she had received bad advice from lawyers, but that she never intended to defraud anyone. He said later that he was considering asking U.S. District Judge William B. Bryant to set aside the verdict on the grounds that the mail-fraud statute was not intended to apply to this type of case.

On two occasions while the jury was out of the room, but before it began deliberations, Bryant told the court that the case against Gamble "sticks in my craw. She doesn't give the appearance of a vicious, venomous individual out to milk people or line her pockets," he said. "She was a pauper; she couldn't even pay her rent." He observed that NIRA had paid Gamble a "messenger's wages" of just $19,000 over 18 months.

He berated the prosecution for bringing a criminal case. "You don't charge somebody with being a thief unless she benefits," he told Fields. "Technically, [Gamble] shouldn't have taken this money and tried to get a business off the ground, [but] civil sanctions should have come into effect. I don't think she's [a] trickster and a swindler."

At that point, Gamble's attorney asked the judge not to let the case go to the jury.

But he decided to let her peers decide after Fields countered that Gamble had lied to her clients.