For a 6dB/octave lowpass filter, I would say power gain is inversely proportional to frequency squared (1/f^2). There's precedent for this notation, for example

For a 6dB/octave lowpass filter, I would say power gain is inversely proportional to frequency squared (1/f^2). There's precedent for this notation, for example pink noise is known as both 3dB/octave and as 1/f.