If you’re interested in learning Gann, you should read the books he authored and the books he suggested you study for background knowledge.
Books Gann Authored
Truth of the Stock Tape and the Wall Street Stock Selector (1923)
Tunnel Thru the Air (1927)
Wall Street Stock Selector (1930)
Fact Facts America, Looking Ahead to 1950 (1940)
45 Years in Wall Street and the New Stock Trend Detector (1949)
The Magic Word (1950)
How to Make Profits in Commodities
How to Make Profits Trading in Puts and Calls
New Stock Trend Detector
Scientific Stock Forecasting or Large Profits on Small Risks
Companion to Truth of the Stock Tape
Gann Commodity Course
Gann Stock Market Course
Books he suggested you read
Most of them a freely available here