Yes, EPON is much simpler in topology. Pretty much Ethernet frames over fiber. GPON encapsulates ethernet frames in GEM Ports, T-CONTs and VLAN IDs, different beast altogether. The problem now is that some fiber routers are EPON-only, GPON-only or switchable. I know for a fact that Nokia routers allow switching of transciever protocols but haven't extensively tested. Not sure how to do it on my Xyxel unit or if it's even possible.
By the way, there is already OpenWrt support for a GPON module here: Support MA5671A SFP GPON
It's a SFP GPON module with MAC, basically a miniaturised router with Lantiq SoC so it's a bit pricey. Maybe they can give us some leads on how to approach this.