Question: On what occasions did Girija Prasad Koirala headed the Nepali Congress and served as the prime minister of Nepal?
Answer: Girija Prasad Koirala headed the Nepali Congress and served as the prime minister of Nepal on four occasions, including from 1991 to 1994, 1998 to 1999, 2000 to 2001, and from 2006 to 2008.