Compute the sum $$$\left\lfloor \frac{m}{n} \right\rfloor + \left\lfloor \frac{2m}{n} \right\rfloor + \cdots + \left\lfloor \frac{(n-1)m}{n} \right\rfloor = \sum_{k=1}^{n-1} \left\lfloor \frac{km}{n} \right\rfloor$$$

I'm told that there's a way to think about this that is similar to the Euclidean Algorithm for gcd. Is there a connection to that?