I actually remain unconvinced by the non-scholarly sources people are providing, authoritatively claiming etymology on scant evidence, and believe they are relying on the self-reported motives of Maverick.

'Gook' is only generally used as a slur, and other uses are obsolete by a century or extremely regional and rare. In 1944 the slur was the predominant usage, particularly around Maverick.

Do you have a more authoritative source?