The Alberta man — named National Geographic's adventurer of the year in 2015 — made the climb Jan 27.
Gadd, 47, was followed by another Canadian, Sarah Hueniken, 34, who grew up near the falls.
The climb was started on the American side of the falls, the National Geographic reported on its website.
"The power of the falls is staggering," Gadd told National Geographic. "It vibrates your intestines and makes you feel very, very small. I've never experienced anything like it."
Gadd says he followed a route almost exactly along the border between the Canadian and U.S. sides.
Gadd is a professional climber who has scaled the glaciers of Kilimanjaro.