You can not use a standard ethernet or GPON SFP - these are normally just media converters which change an electrical bitstream into an optical one and vice-versa.
There are some third-party SFPs which include the functionality of a GPON ONT implementing all of the protocol conversion and processing required. As a ONT registers itself with the ISP equipment you have to duplicate the identity it provides in the third-party SFP, it almost certainly will not implement the same management interface which the ISP may detect and block your service if they do not permit the use of third-party devices. There are many threads on the foruk, for example http://forum.mikrotik.com/t/usage-gpon-module-sfp-in-spain/104807/1
If your ISP supports bridge mode in their ONT that is a safer option - it handles the GPON interface and provides an IPoE or PPPoE WAN connection to you router allowing you to control the LAN side addressing and any port forwarding desired.