The Justice Department's investigation of price-fixing by an international uranium cartel entered its final stages yesterday with a misdemeanor conviction of Gulf Oil Corp. and the filing of criminal contempt proceedings against a California uranium broker.

Gulf pleaded "no contest" to a criminal information and was fined $40,000 yesterday in U.S. District Court in Erie, Pa., for its part in an alleged conspiracy that boosted the price of uranium from $6 a pound to $41 a pound in the early 1970s.

In a separate action, a Justice Department petition in U.S. District Court in Washington charged that George White Jr., executive vice president of Nuclear Exchange Corp. of Menlo Park, Calif., destoryed business records he knew had been subpoenaed by a grand jury.

Both charges grew out of an investigation by a federal grand jury here whose term expired in mid-May. No further criminal actions are expected from that investigation of uranium price-fixing, Justice Department officials said yesterday.

The Gulf plea avoided a lengthy and expensive court defense, a company spokesman said, while not jeopardizing its position in a number of civil suits filed by purchasers and utilities who claim they were overcharged million of dollars by the cartel.

While a "no contest" plea is the equivalent of a guilty plea, it doesn't have a carryover effect on civil cases.

Gulf could have been fined a maximum of $50,000. But the company spokesman quoted U.S. District Court Judge Gerald J. Weber as saying he felt the firm "is entitled to some symbolic recognition for coming forward and resolving this issue and saving all parties a long and costly trial."

A New York state investigator this week challenged the original Justice Department misdemeanor charge as too lenient. He said the government had evidence enough to support a felony charge, punishable by a $1 million fine.

In the related case against California businessman White, the Justice Department said his firm admitted in a September 1976 affidavit that documents had been destroyed after the subpoena was received.

White said in a brief telephone interview that "it wouldn't be appropriate for me to comment now." The subpoena asked for documents relating to agreements about minimum prices for uranium sales.

White's firm, known as Nuexco, arranged transactions between buyers and sellers of "yellowcake," the raw uranium used to make fuel for nuclear power plants. The firm also publishes a newsletter containing a price index on yellowcake.