Bill Gross, manager of the world's largest bond fund, said Wednesday that income, rather than capital gains, would drive future investment returns as a result of low global economic growth.

"Capital gains from almost all asset classes are approaching dusk. Low but relatively dependable income will be the market's future driver," Gross, the chief investment officer of Pimco, said in his latest monthly letter to investors titled "Goodnight Vietnam."

Income is the interest paid on an asset, whereas capital gains are increases in the asset's price.

Gross, who predicted in May of last year that the roughly 30-year bull run in bonds had likely ended, said Wednesday that since bond yields were headed higher while global economic growth would remain low, gains in asset prices would be tough to achieve.

"So as yields have bottomed and are now expected by the markets to gradually rise, it's down to growth, and growth is a question mark," Gross said. He said a lack of "aggregate demand" and an overabundance of "aggregate supply," partly as a result of over-indebted consumers, would continue to weigh on global growth rates.

Gross' views are closely watched because his flagship Pimco Total Return Fund is the world's biggest bond fund, with $225 billion in assets.

Pacific Investment Management Co. altogether had $1.97 trillion in assets as of June 30, according to the firm's website.

Gross, whose flagship fund had a 47 percent exposure to U.S. government-related assets in June, said high-quality Treasury and corporate bonds were "fairly priced, but not cheap." He reiterated that the Federal Reserve would likely not raise interest rates until mid-2015, and said that investors should own bonds and "an average proportion of stocks."

Gross' comments are in line with Pimco's "New Neutral" outlook for the global economy, which the firm introduced in May. New Neutral suggests the global economy is transforming from a post-financial-crisis recovery period called the New Normal in 2009 toward stability characterized by modest economic growth over the next three to five years.

Gross told Reuters on July 18 that the yield on the benchmark 10-year U.S. Treasury note belonged in the 2.50 percent to 3 percent range "now and for the next few years" if the Fed remains accommodative.

Gross' Total Return Fund is up 3.62 percent so far this year, trailing 77 percent of its peers, according to data from Morningstar. The Newport Beach, California-based Pimco is a unit of European financial services company Allianz SE.