From the latest, and alas last, edition of Safire’s Political Dictionary (full disclosure: he said some very flattering things about me in the Prolegomenon):
-gate construction A device to provide a sinister label to a possible scandal.
After WATERGATE, a scandal in France dealing with the adulteration of Bordeaux wines was promptly dubbed “Winegate.” This led to the adoption of the -gate suffix as a scandalizer in other fields.
When it became known in 1976 that an investigation had been launched into corruption of members of the U.S. Congress by the South Korean CIA, this writer started referring automatically to “Koreagate.” When mayoral candidate Mario Cuomo attacked New York Mayor Abe Beame for seeking to suppress an SEC report on the finances of the BIG APPLE—New York City—he called it “Applegate.” More aptly, charges leveled at Congressman Daniel Flood in 1978 were called “Floodgate,” and charges that some contractors were double-billing the government reminded those acquainted with British English of the possibilities of double-billingsgate. The formulation with the -gate suffix is too useful to fade quickly.