reading the frequency and the magnitued from a GH bode plot
If it's a unity feedback system (\$\small H=1\$), express \$\small GH\$ as a complex number at the frequency of interest: \$\small GH=x+jy\$, then the closed loop response at that frequency is \$\small T(j\omega) =\large \frac{x+jy}{(1+x)+jy}\$. Then determine magnitude and phase angle in the usual way.

If it's not unity feedback, you need to know \$\small G(j\omega)\$ or \$\small H(j\omega)\$ to work out the system frequency response.