Although GPON is shared, it is not half duplex! I'm not exactly sure why the asymmetrical version won out, perhaps due to cheaper optics back in the day?
Latest iterations (NG-PON2 and XGS-PON) are 10G symmetrical, and XGS-PON is now cheap enough that GPON doesn't make much sense. AT&T Fiber in the US has been deploying it for the last few years.