A former store manager for the General Services Administration told a federal judge here yesterday that he accepted nearly $6,000 worth of gifts from a private firm in return for falsifying the store's supply invoices to benefit the firm.

Eugene D. Shadle Jr., former acting manager of the GSA self-service store at the Housing and Urban Development Building in Washington, was the 12th person to plead guilty to conspiring to defraud the federal government in the GSA bribery scandal.

So far, only one of the 18 persons indicated Sept. 29 has pleaded not guilty to the charges. Five others will be arraigned later this week.

Prosecutors said the former GSA store manager and two Baltimore firms connected in the first round of GSA indictments defrauded the taxpayers of more than a million dollars with bribery and fraud schemes.

Shadle. 36, of Waldorf, was the assistant manager of the GSA store at the Nazy Yard Annex in Washington when the indictments were returned. He was suspended from the GSA and said he now wroks as the assistant parts manager for a car dealer in Landover.

Shadle told Judge C. Stanley Blair he accepted cash, two color television sets, an air compressor, clothes, liquor, jewelry and a sandblaster from the film of James Hilles Associates in return for signing invoices for supplies of file folders which he never received.

Assistant U.S. Attorney M. Clements said Shadle is cooperating with the continuing GSA investigation. Sentencing will be delayed for several months. The maximum penalty for the conspiracy charge is five years in jail and a $10,000 fine.

In another case, a 26-year-old Alexandria man pleaded guity yesterday to three counts of theft of government property after using a GSA credit card that he had borrowed.

The man, Bruce L. Jefferson, of 5380 Holmes Parkway, entered the plea before U.S. District Judge George Hart in Washington.

The original indictment covered the theft of items worth an estimated $36,500 according to Assistant U.S. altorney David Krakoff. However, a new indictment that was brought yesterday against Jefferson reduced the number of counts from 169 to 47, and Jefferson then pleaded guilty to three counts involving a total amount of $798.