The noun gear refers to several things. First, it’s the equipment we need for a sport or event. A gear is also a wheel with teeth that interlocks with another gear to transmit motion in a machine.
The background of gear as "equipment" dates back to around 1200 and comes from the Old Norse gervi, "apparel." The word gear in machinery became common in the early 19th century, although there is evidence that the word came to mean "toothed wheel" as early as 1520. Gear entered English slang around 1951, meaning something stylish. This use was popularized in the 1960s, under the influence of British pop music.