The fund's monthly performance beat 41 percent of peers, Morningstar data showed. The fund, which has roughly $237 billion in assets, fell 1.92 percent in 2013 after bond prices dropped on fears the Federal Reserve could reduce its monthly bond-buying stimulus.
Gross's fund likely benefited from a rise in Treasuries prices in January, said Todd Rosenbluth, director of mutual fund research for S&P Capital IQ. The fund's largest holding of 45 percent was in U.S. government-related debt as of Dec. 31, according to data on the Pimco website.
Benchmark 10-year U.S. Treasury notes notched their biggest gain in 20 months in January on weaker-than-expected U.S. economic data and after a rout in emerging market assets spurred safe-haven bids.