Why did they call Ronald Reagan "The Gipper"?

Ronald Reagan was a professional actor before he was elected president, and he portrayed American college football player George "The Gipper" Gipp in the 1940 film "Knut Rockne, All American." The phrase "Win one for the Gipper" was purportedly said by Gipp on his hospital bed.

"Knut Rockne, All American" presents the life and career of Notre Dame football coach Knut Rockne. Reagan portrayed football player George Gipp. In the film, Gipp was stricken with a terminal illness, and he urged Rockne to "win one for the Gipper." Reagan used the line as a political slogan, most notably at the 1988 Republican Convention, when he urged his vice president, George H.W. Bush, to go win one for the Gipper.