It doesn't necessarily mean the network is not technically capable, they probably just don't have a plan with faster speeds.

GPON is 2.4/1.2 Gbps and it's shared with multiple users so it's incredibly unlikely that any part if the core infrastructure is limited to Gigabit. Only the CPE is.

You can deploy 2 Gbps service on GPON though the quality will be low-ish, or you can run multiple versions of xPON standards on a single fiber network and offer > 1 Gbps to users who you switch to XGPON2. Others stay with existing CPE and plans and GPON as if nothing changed.