As Allison wrote, the gain depends on the operating conditions other
than just collector supply voltage. However, a reasonable rule of
thumb if you don't change the operating class or circuit topology, but
do adjust the output matching network as appropriate, is that the gain
would change by 3dB per doubling of supply voltage (or -3dB for halving
it). Think of it this way: you get the same current, and you're only
changing the voltage. Power equals voltage times current. Of course,
you're limited by the transistor's breakdown voltage, and too much gain
can lead to instabilities. And at very low voltages, you need to
consider the transistor's saturation voltage.

Cheers,
Tom