In a private ceremony in the Oval Office yesterday, President Reagan presented the family of slain congressman Leo J. Ryan with the Congressional Gold Medal, Congress' highest civilian honor.

After the presentation, the family, including the late congressman's mother, Autumn Ryan, and four of his five children, gathered outside the White House to talk about Ryan's life. Ryan, who represented several cities in the San Francisco Bay Area, was killed in November 1978 in Guyana by followers of Jim Jones, the leader of the Peoples Temple cult. The congressman had gone there to investigate the encampment, whose 913 members committed mass suicide the day of Ryan's murder.

The afternoon ceremony was described as warm and simple by Autumn Ryan, who said the president reminisced about the years when Ryan was a member of the California State Assembly and Reagan was governor. "My feelings were mixed," she said. "I knew Leo was a brave man." But she added quickly that she wasn't bitter that her son could not get government officials to go to Jonestown. She recalled their last conversation: "He told me, 'I made a promise to the relatives, I can't get anything out of the State Department, I will see you Saturday or Sunday.' "

Also attending was Shannon Ryan, 32, the oldest daughter of the congressman. She is now a follower of the Indian guru Bhagwan Shree Rajneesh and a resident of his Oregon commune. Her father, she said, would be open-minded about her life. "He would come to Rajneeshpuram and find out if it was good or bad before he judged. It is a beautiful, clean and crime-free place," said Shannon Ryan, who has adopted the name Ma Amrita Pritam.

For two years, Erin Ryan and Patricia Ryan, two other daughters, worked to get Congress to vote for the medal. They walked the halls, recalled Erin Ryan yesterday, securing the 218 cosponsors for the bill.

The family selected the words of Greek historian Thucydides for the medal inscription, which says, "But the bravest are surely those who have the clearest vision of what's before them, glory and danger alike, and yet not withstanding, go out and meet it."

Said Patricia Ryan, "He had been a history teacher, and I don't know if he ever applied that to himself. But we thought it fit."