Aside from the football team San Francisco 49ers, this refers to the people who flocked to California between 1849-1900. Overseas immigrants, particuarily from China, would arrive by boat in San Francisco.
First word of the gold rush actually started in 1848, but by 1849 the entire world knew about the gold rush in California. It's estimated that by 1855 over 300,000 people had immigrated to California in search of gold.
The football team's cheerleaders are called the "Gold Rush".