To be able to effectively use gyraf's equation, you need to have a known small a.c. voltage and apply it to the input. Most meters don'r work well at low levels so start with something you can measure and divide it down with some appropriate resistors. For example, take, say, a 1V rms 1kHz signal, as measured with your certified a.c. voltmeter, and feed it to a 10k resistor in series with a 10 ohm resistor to common. At the junction of the two resistors you have 1 mV rms, which is at a low enough impedance (about 10 ohms) that you can safely assume it doesn't change much when you connect it to your input. If it's a balanced input you can tie one side to common.

Measure the output of the unit under test with the same voltmeter. Per gyraf's formula, divide that reading by (for this example) 1 mV. Take the (base ten) logarithm and multiply by 20. This is the gain expressed in dB.