Note: the Ngram Viewer treats quotation marks literally. Replaced I'm going to with I ' m going to to match how we processed the books. Replaced I'm gonna with I ' m gonna to match how we processed the books.
Search in Google Books: 1800 - 1906 1907 - 1940 1941 - 1949 1950 - 1992 1993 - 2000 I'm going to English 1800 - 1943 1944 - 1990 1991 - 1994 1995 - 1997 1998 - 2000 I'm gonna English 1800 - 1813 1814 - 1857 1858 - 1871 1872 - 1958 1959 - 2000 I will English 1800 - 1830 1831 - 1895 1896 - 1907 1908 - 1973 1974 - 2000 I am going to English