Lafayette, La. — A defrocked Roman Catholic priest who had admitted having sex with dozens of boys pleaded guilty today to pornography involving children and other charges and was sentenced to 20 years at hard labor.

The plea, part of an agreement with prosecutors, came as trial was to open for the former priest, Gilbert Gauthe.

As part of the bargain, prosecutors dropped a charge of aggravated rape that carried a mandatory life sentence.

In exchange, Mr. Gauthe pleaded guilty today to 11 counts each of child pornography, crime against nature and contributing to the delinquency of a minor. The 20-year sentence carries a provision of no parole.

The District Attorney, Nathan Stansbury, said families of the children involved had agreed to the sentence.

Paul Hebert, a lawyer who has filed civil suits on behalf of some of the children, said the families involved wanted to avoid the publicity of a trial and that they felt ''under the circumstances that this is the best that they could do.''

Insanity Plea Abandoned

Mr. Gauthe, 40 years old, made no statement before the court. He had previously pleaded not guilty by reason of insanity and had been free on $250,000 bond. Mr. Gauthe has been under treatment in a mental hospital run by the church.

He was pastor from 1977 to 1983 at St. John's Church, in the small town of Henry in Vermilion Parish. In a 78-page deposition, he said that while he was a priest, he engaged in oral sex and sodomy with young people, including some altar boys and some girls. He said he could remember 35 of the boys.

He said the encounters took place in his van, at the rectory, at a camp or in a confessional. Mr. Gauthe said he had often photographed the episodes but had burned the pictures.

Accusations against Mr. Gauthe, who was relieved of his priestly duties in June 1983, had sharply divided the heavily Catholic region and prompted millions of dollars in lawsuits against the church.

Officials in the Diocese of Lafayette, which covers eight parishes of Louisiana, have refused to discuss the case, referring inquiries to church lawyers.

The church acknowledged liability in the damage suits, and the church and its insurance companies have reportedly paid more than $5 million for lawyer's fees and six settlements. Russell Gaspard, clerk of court for Vermilion Parish, said nine suits were pending.