Under the null hypothesis that there is no preference the mean number who bought GM is 566, and the SD of the number that bought GM is ~=16.8 and the distribution is B(1132,0.5)

Now use a one sided z-test (the numbers are large, so it is OK to assume normality, but use a continuity correction anyway).

(in fact there is no need for a formal test, the z-score is ~6.8 so we reject the null hypothesis without any further thought)

CB