Bill Gross, the founder of Pacific Investment Management Co. and an avid stamp collector, has given $8 million to the National Postal Museum in Washington to create a gallery that will exhibit some of his most prized acquisitions. The 12,000-square-foot William H. Gross Stamp Gallery will house three rare items on loan from his collection, including a letter-carrying envelope from the Pony Express service and a block of four 1918 “Inverted Jennys" stamps...