Gambit has changed. Its dictionary-approved definitions include (1) an opening chess move in which a piece is sacrificed in exchange for a favorable position, (2) an opening maneuver, and (3) a remark opening a conversation. Each of these senses, you’ll notice, involves the start of something. Because gambits are by definition opening, the phrase opening gambit would be a redundancy.
But in today’s English, most writers who use gambit disregard the opening part of its definitions, using it simply as a synonym of maneuver, usually connoting cleverness or strategic planning. The element of sacrifice, which is part of the chess-related definition and its figurative extensions, is also mostly dispensed with. This use of gambit is rife and appears in all sorts of writing from throughout the English-speaking world, so there is no doubt that it is now accepted.
Of course, careful writers can still use the word in its old senses, but if its original meaning were to become lost to most English speakers, then using it in its old senses might only cause confusion.
If we give precedent to the dictionary definitions of gambit, then these sentences don’t need opening:
The bill taking shape will be the opening gambit in what is expected to be a session-long debate over the future of gambling in the state. [Washington Post]
But it is merely an opening gambit in the game now being played out in Edinburgh and London. [Politics.co.uk]
But, as used in these sentences, gambit is not an opening move but a clever or strategic maneuver:
It was his latest gambit in a complex power struggle set off by the American raid that killed Osama bin Laden in May. [New York Times]
But it’s the closing gambit we love the most. [Telegraph]
It is a surprisingly simple gambit that will complete the process of corporate globalization by bringing it full circle back to its birthplace in the former “first world.” [Fiction's Present]
But Zack’s final gambit – turning Jackie’s words against Alicia to give Nisa an all-access pass? [AV Club]
Gambit is sometimes used in its traditional senses, but examples such as these are not easy to find:
As a professional gambit, it set a certain tone which they have followed assiduously ever since. [Guardian]
Another common gambit is to begin a story with a character who is numb with grief because someone close to her has died. [Wall Street Journal]
Gambit vs. gamut
Incidentally, in looking for examples for the above, we found a few instances of gambit used in place of gamut—for example:
Players, coaches and the gambit of sports figures will answer readers’ questions in The Denver Post’s “Fan Mail” feature. [Denver Post]
QBE has operations in 49 countries, so it has been exposed to the full gambit of calamities experienced in what has been the worst year for insurers on record. [Dow Jones Newswires]
Web searches for the phrase “full gambit of” reveal that this confusion happens rather often. Gamut, of course, is the word for a complete range or extent.