I am reading this article and looking at eq. (4). I am not sure how the expressions in terms of the viewing angle $\iota$ and the phase $\Phi$ are achieved. It looks to me like it has nothing to do with general relativity but with geometrical definitions. Let the gravitational wave amplitude be $$ h_0 \equiv \frac{4c}{D} \left( \frac{G \cal{M}}{c^3} \right) \Omega^{2/3} .$$ Then the expressions are
$$h_+ = h_0 \cdot \frac{1}{2} (1+\cos^2\iota)\cos 2\Phi(t)$$
$$ h_x = h_0 \cdot \cos\iota \sin 2\Phi(t).$$
The angles are related to how we observe the waves, and I am not sure how they come about.