Error Amplifier Gain
The graph's displayed gain at DC does match the given equation. But the graph expresses gain in dB.

Consider the case of \$R_L = 30k\Omega\$.

$$A_v = gm * R_L = (1.5 mS) * (30 k\Omega) = 45 V/V$$
$$A_v(dB) = 20 log(A_v(V/V)) = 20 log (45) = 33 dB$$

The line for \$R_L\ = 30 k\Omega\$ conveniently intersects the vertical axis around 33dB.