The most recent auction, the first to include U.S. issues, went off on April 9, here's Nightly Business Report with the details:
Here's the Auctioneers announcement via StampAuctionNetwork:
Robert A. Siegel Auction Galleries, Inc. Sale - 1041
As Mr. Gross and his wife, Sue, have so generously done with all of their stamp proceeds, the money from this auction will be immediately donated to two major charitable institutions—Doctors Without Borders/Medecins San Frontieres and Millennium Villages Project at the Earth Institute.
The auction is held in collaboration with Charles F. Shreve and Tracy L. Shreve.
Second Session (lots 124-375): Tuesday, April 9, 2013, at 1:30 p.m.
Robert A. Siegel Auction Galleries, Inc.
60 East 56th Street, 4th Floor
New York, New York 10022
Phone: (212) 753-6421
Fax: (212) 753-6429
To request more information by email. stamps@siegelauctions.com
Visit our Home Page at Robert A. Siegel Auction Galleries, Inc.
Finally, the press release with many more details:
PIMCO Founder Bill Gross Will Sell Some Of His Historic U.S. Stamps in New York City Charity Auction