IIR - How the gain is calculated?
Apparently it's normalized so that the gain at Nyqusit is 0 dB. Potentially they just normalize to the maximum being 0 dB. Gain at Nyquist is given by

$$H(\omega = \pi) = \frac{b_0-b_1+b_2}{a_0-a_1+a_2}$$

whereas gain at DC is

$$H(\omega = 0) = \frac{b_0+b_1+b_2}{a_0+a_1+a_2}$$

Both values are real.