The "glass fiber modem" is an inherent part of the GPON network. These are complicated. The "P" stands for "passive". Yours and and up to 127 other houses are all on the same "light domain" i.e. the downstream is passively split, and the upstream is passively combined, in optical boxes that don't even have electrical parts.

This needs crazy accurate timing for the upstream. The head end needs to know the exact delay to your particular box to give it a "grant" to transmit at exactly the right time so transmit bandwidth is not wasted by idle time or multiple boxes transmitting at the same time and corrupting each other.

You don't want brand X modems with dodgy configurations in this. Of course as a consumer you'd want "as little modem as possible" i.e. just give me an ethernet port running DHCP or PPPOE and let me do the rest.