Cavity gain of a laser

In textbook, the cavity gain of laser is given by:
(1-R₁)(1-R₂)G_s/
(1-(R₁R₂)^?G_s)2+4(R₁R₂)^?G_ssin2Φ
Where Φ is a function of the light frequency.
I don't understand if Φ is a function of f, which means the gain vary endlessly against the frequency of light, however, the cavity support infinite no of longitudinal mode, then how the laser can choose to emit only a specific range of frequency of light?

well, the simple answer is: they don't emit at a single frequency! Look at the spectrum of a standard laser diode..you will see a hump of frequencies!

Only (in semiconductor lasers) if you use distributed feedback type (i.e. have a bandpass filter type of reflector mirror) will you get MOST of the energy in one spectral line.