The word gate is a good old English word, the form being geat in Old English (plural geatu) and whose meaning was ‘an opening or entrance’. A watergate (from the 15th century) is a channel for water; the first element of the compound, water, is also a native English word.
Its use as a suffix is fairly recent, dating to the infamous Watergate incident of the Nixon administration. The shorthand referent for this major political scandal became the name of the hotel itself — the location of the crime’s execution. Pulling the compound apart, the second term (-gate) has become a highly productive ending, now carrying the meaning of ‘scandal’. Amazing little cognitive move, isn’t it? Wikipedia lists dozens of coined terms for scandals that now use this term, including a few cases of fictional scandals from sitcom episodes.
Here are just a few among the more famous recent scandals:
Note how the semantics of ‘political scandal’ has generalized to ‘personal scandal’ (Tigergate) and ‘financial scandal’ (Angelgate). The form’s usage is robust enough to include variants for a single incident (Tasergate/Troopergate). It will be interesting to see whether the suffix’s course through English becomes even broader in time to encompass more general incidents that don’t convey the notion of ‘scandal’. Will the suffix -gate expand to include affairs and events that don’t convey a pejorative sense, perhaps including some with a positive meaning?