What is voltage in dB:
Voltage in dB=20*log10(voltage in volts)

Example 1:
20*log10(1 volt)= 0 dB

Example 2:
20*log10(2 volts)= 6 dB

Example 3:
20*log10(0.5 volts)= -6 dB

Like someone said above, the fact that you have negative dB values means that you have less than 1 volt.

That's relative to a 1V input. dB is a relative gain value. Thus a negative dB value is gain of less than one. With a 1V input then it's an output voltage less than 1V. But if you have a 1mv input, for example, then it would be an output less than 1mV.

 

That's relative to a 1V input. dB is a relative gain value. Thus a negative dB value is gain of less than one. With a 1V input then it's an output voltage less than 1V. But if you have a 1mv input, for example, then it would be an output less than 1mV.

My point is that I don't think OP knows how to convert "regular" values to values in dB.

Let us use what you have posted to expand the lesson.
Let us say:
Input voltage (Vin) is 1 volt.
Output voltage (Vout) is 0.5 volts.

Then Gain is:
Gain=Vout/Vin=0.5/1=0.5 (Gain has no units)

What is Gain in dB?
Gain in dB=20*log10(0.5)= -6 dB

So the Gain is a positive number, in this case the input signal will be reduced by half, it will be attenuated. But when we look at the Gain in dB, we see a negative number. The OP, in my reading, did not understand these negative numbers.