GPON is time divided, not frequency divided. You have one wavelength for Tx and one for Rx and everyone on the PON has to split it.

It is certainly POSSIBLE to do WDM on a PON, but you need separate matched pairs of transceivers for each connection which kind of defeats the purpose (you now need a mux/demux at the end which adds more attenuation to the line, as well as more expensive electronics as you went from needing one GPON port to N Gbe ports). As such, wavelength division is typically used over point-to-point links to get more discrete links over a single fiber (or pair).