Gates, 63, served as CIA director from November 1991 to January 1993 during the presidency of Republican George Bush, the current president's father.
He joined the CIA in 1966 and spent nearly 27 years as an intelligence professional.
First nominated as CIA director in 1987 by President Ronald Reagan, Gates withdrew amid questions over his and the CIA's role in the secret sales of arms to Iran and the diversion of profits to Nicaragua's Contra rebels.
In hearings in 1991 Gates admitted mistakes and said he should have done more to get at the truth.
Had controversial confirmation hearings for CIA director including charges he hid the truth about Iran-Contra affair from Congress when that scandal was breaking.
He served as deputy director of Central Intelligence from 1986 to 1989 and as deputy national security adviser for president George Bush at the White House from 1989 until 1991.
He was also a member of the National Security Council.
Currently Gates is the president of Texas A and M University in College Station, Texas, and is a member of the bipartisan Iraq Study Group.
That group, headed by Republican former secretary of state James Baker [another Texan] and Democrat Lee Hamilton, is working on recommendations to present the government about the Iraq war.
The group is expected by the end of the year to issue alternative ideas for a way forward in Iraq.