Convert the input voltage to Volts.

$$V_{in} = 120 \text{ mV} = 120 \times 10^{-3} \text{ V} = 0.120 \text{ V}$$

The output voltage is given as $V_{out} = 6 \text{ V}$.

$$A_{V,dB} = 20 \log_{10} \left( \frac{V_{out}}{V_{in}} \right)$$

Substitute the values:

$$A_{V,dB} = 20 \log_{10} \left( \frac{6 \text{ V}}{0.120 \text{ V}} \right)$$

$$A_{V,dB} = 20 \times 1.69897$$ $$A_{V,dB} = 33.9794 \text{ dB}$$

Rounding to two decimal places:

$$A_{V,dB} \approx 33.98 \text{ dB}$$

The voltage gain of the amplifier in decibels is $\text{33.98 dB}$.