A peek inside bond guru Bill Gross’ multimillion-dollar stamp collection that he's selling

• Bond guru Bill Gross is exiting one of his long-standing investments and is giving all the proceeds to charity.
• That investment is rare U.S. stamps, which Gross has been collecting since 1993.
• "It's better to give other collectors a chance to own some of these stamps and to use the proceeds for philanthropy," Gross says.

Bond guru Bill Gross told CNBC he is exiting one of his long-standing investments and is giving all the proceeds to charity.

That investment is rare U.S. stamps, which Gross has been collecting since 1993.

"There are limits at some point to a collector's portfolio because if pursued long enough and admittedly with enough money it's possible to fill in all the spaces in an album – in other words to get them all," Gross said in an interview with "Power Lunch."

"I figured it's better to give other collectors a chance to own some of these stamps and to use the proceeds for philanthropy."

Adam Jeffery | CNBC

The largest known unused block of 10-cent 1847 first-issue stamps estimated to fetch between $500,000 and $750,000 at auction.

Donating proceeds is something he's done before. "Perhaps $30 to $40 million already," said Gross, who co-founded Pimco and now manages the Janus Henderson Global Unconstrained Bond Fund.

He plans to sell his collection of about 3,000 stamps through a series of auctions, with the first one expected this fall.

Among the treasures Gross has is a letter from the Pony Express.

Adam Jeffery | CNBC

The historic Pony Express cover signed by co-founder William H. Russell estimated to fetch between $75,000 and $100,000 at auction.

He also has a block of six of the first U.S. stamps, which were issued in 1847, worth between $500,000 and $750,000.

Charles Shreve, director for Robert A. Siegel Auction Galleries and Gross' stamp advisor for the past 25 years, told CNBC that block of stamps somehow ended up in a Bible, where they remained for "dozens and dozens of years." It remains the only intact block of six of the first U.S. stamps, he said.

Also included in the collection is a set of upside-down steamship stamps, the largest known intact multiple of any inverted stamp error, said Shreve. "Stamp collectors love errors," he said.

Adam Jeffery | CNBC

The largest known multiple of any inverted center postage stamp error expected to fetch between $300,000-$400,000 at auction.

While the auction is "at the high-end level," Shreve said he expects a good turnout.

"There will be hundreds of people around the world who will be bidding. There will be a live auction but people will also be able to bid live on the internet," he said.

The first set of stamps hitting the auction block are worth an estimated $10 million, while the entire collection is worth tens of millions.

Gross currently holds the record for the biggest single-day stamp auction in 2007 when he auctioned off his collection of British stamps for $9.1 million.

— CNBC's Kevin Flynn contributed to this report.