From Traderpedia

Definition: Willian D. Gann was a mathematician cum astrological trader of mixed repute who lived and traded around the late 19th century.

Some hold Gann's mathematical solutions and other more unconventional approaches to the market in high esteem, while others allege he was a fraudster.

Gann fans form a standard indicator in many charting software packages.

Gann charts are a less widely known Gann technique - see the picture below... some would say that if you draw enough lines on a chart, some of them are going to line up with something.