BJT Common Emitter Positive Power Gain
To expand on Alex's and W5VO's comments:

An inverting amplifier does not imply a negative power gain. Power is the absolute value of current times voltage through the load, \$P = \left|IV\right|\$ (an amplifier producing negative power has issues).

For instance, if I was to design an inverting buffer (gain of -1), you could also express that as a gain of 0dB with a phase shift of 180º.

However, this is completely independent of power gain.

Let's say the input impedance of the inverting buffer is 1MΩ, and the input signal is 1V. Then the power in is \$P_{IN} = 1\text{uW}\$.

Let's say the output has zero output impedance and it is driving a load of 10Ω. So the load sees -1V, and current through the load is -100mA.

However, the power dissipated in the load is \$P_{OUT} = \frac{V^2}{R} = \frac{(-1V)^2}{10\Omega} = 100\text{mW}\$, a positive quantity.

The power gain is \$\frac{P_{IN}}{P_{OUT}} = 1E5 = 50\text{dB}\$, a very large positive quantity.

I'll leave it to you to work out the power gain for your amplifier, but the point is that the sign of the voltage gain has nothing to do with the power gain.