Why is $f(x)= \frac{1}{x}\sin\left(\frac{1}{x^3}\right)$ not Lebesgue integrable but Gauge integrable?

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Studying Lebesgue Integration and have come across the following function on Wikipedia: $$f(x)=\frac{1}{x}\sin\left(\frac{1}{x^3}\right)$$ I was wondering why the above function is not Lebesgue integrable? And why is it Gauge integrable?