Note that since $f(x) = \dfrac{\sin(ax)\sin(bx)}x$, we have $f(x)$ to be odd. Hence, we have $f(-x) = -f(x)$. Hence, we have
$$G(m,n) = \int_{-m}^{-n} f(x)dx + \int_{n}^{m} f(x)dx = 0$$
Hence, the principal value is zero, i.e.,
$$\lim_{n \to \infty} \lim_{m \to \infty} G(m,n) = 0$$
Note that since $f(x) = \dfrac{\sin(ax)\sin(bx)}x$, we have $f(x)$ to be odd. Hence, we have $f(-x) = -f(x)$. Hence, we have $$G(m,n) = \int_{-m}^{-n} f(x)dx + \int_{n}^{m} f(x)dx = 0$$ Hence, the principal value is zero, i.e., $$\lim_{n \to \infty} \lim_{m \to \infty} G(m,n) = 0$$