Got mine as well:

I think I got this since I played like crazy last week/month, hence wagered more than usual. Maybe a 5%-10% gauge filled in a week/month will trigger this? Or perhaps if the cumulative gauge filled 40%-50% (mine was 42%).

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Anyway, Gates of Olympus Super Scatter, normal spin:
https://www.pplink.social/9ul0QxZ2Km