To provide an answer:
Let's consider a SISO system, where the transfer function of the plant is given by $G(s)$, and the feedback gain, is $H(s)$. Let's also assume that only a proportional control with gain $K$ is to be used.
Then, the closed loop transfer function is given by:
$$\frac{Y(s)}{R(s)}=\frac{KG(s)}{1+KG(s)H(s)}$$
where $Y(s)$ is the plant output, and $R(s)$ is the reference signal input. The characteristic equation is $1+KG(s)H(s)$.
As you know, the Root Locus method plots the roots of the characteristic equation - which are the closed loop poles - as a function of gain $K$, where $K \ge 0$, assuming negative feedback is required. Now for unity feedback, $H(s)=1$, and thus the characteristic equation is given by $1+KG(s)=0$.
Now, let the plant transfer function be given as $$G(s) = \frac{N(s)}{D(s)}=\frac{(s+z_0)(s+z_1)...(s+z_m)}{(s+p_0)(s+p_1)...(s+p_n)}$$ where $m \le n$.
In this case, the characteristic (or closed loop poles) equation is given by:
$$\therefore 1+ KG(s)=1+ K\frac{(s+z_0)(s+z_1)...(s+z_m)}{(s+p_0)(s+p_1)...(s+p_n)}$$
$$\Rightarrow K=-\frac{(s+p_0)(s+p_1)...(s+p_n)}{(s+z_0)(s+z_1)...(s+z_m)}$$
Finally, since the characteristic equation has real coefficients, any complex poles will occur as conjugate pairs - which is why the Root locus plot is typically symmetric about the real axis.
Hence, once the desired closed loop pole, $s_0$ is calculated based on the required control system performance criteria (rise time, maximum overshoot, settling time, etc.), the desired value of the (proportional) control gain can be found as:
$$K=\left. -\frac{(s+p_0)(s+p_1)...(s+p_n)}{(s+z_0)(s+z_1)...(s+z_m)}\right|_{s=s_0}$$
Again, since negative feedback is used, we want values of $K \ge 0$.
It should be noted that for a second order realizable plant with proportional gain and unity feedback, the closed loop transfer function will also be of second order, with the characteristic equation in the form $s^2 + 2\zeta \omega_n s + \omega_n^2$.
