Is there any GPON ONT are supposed by openwrt/LEDE?
I will guess that you meant "supported."
I surmise not; but check the Table of Hardware.
In my nation ONTs are owned by the ISP (my ISP's is delivered Internet in MoCA coax Ethernet or in RJ-45 Ethernet); but I understand in some nations like Germany, they are owned by the user.
Yes ONTs are owned by ISP. But could buy one from eBay/taobao and try config it.
DO you have [legal] access to the drivers of the network interface card if you were able to boot the CPU?
I didn't know there were any Open Source MoCA drives, for example.
In the USA you can (and should) buy your own cable modem, but the cable company still controls its firmware. So you should run your own router and use the modem in bridge mode just as a modem.
ONTs are likely a very similar situation.
Apart from x86 hardware I think there are Marvell devboards like MacchiatoBin and ClearFog, they either have it or have a GBIC slot (then you would have to buy the right module and they are not cheap). I don't know about the actual driver support (of the slot and of the module).
Imho it would be much better to look for a "modem" device that supports bridge mode, that's usually the best way to deal with devices that in 99.99% of the cases have only proprietary drivers. Then any device using Ethernet can use pppoE protocol to control them and be the actual router.
In case the OP is trying to set up a fiber network of their own, as opposed to trying to connect to an ISP network, you can definitely do this using a managed switch with an SFP port and an appropriate plug-in. Examples are:
Those examples have 2 SFP ports, then you buy an appropriate transceiver, such as maybe https://www.amazon.com/TRENDnet-Multi-Mode-Mini-GBIC-Compatible-TEG-MGBSX/dp/B0009JR5SM
Then you use whatever you like for a router running OpenWrt. There's really not a need to actually have OpenWrt control the optical hardware.