Game theory is typically quite analytical and thus constitutes a stronger signal than most undergraduate field courses. However, at least a semester (if not a full year) of econometrics and/or statistics is very important.
Your primary objective should definitely to have a decent amount of coursework in each of economics, mathematics, and statitistics/econometrics. Economics is ironically probably the least important of these in terms of preparation. Once you have that taken care of, I'd worry more about your preferences and interests and securing strong letters of recommendation from faculty members.
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