hello, have similar situation
i managed to register external gsm gateway with user account on FusionPBX. Then in outbound route i've put in last bridge statement: user/<login@fusiopbx-ip>
so call goes out to/via this extension
but in my case calls goes out as <login@gateway-ip> and my gateway considers it as SIP-call and makes outbound GSM call to number <login> i.e. to gsm number 11223344
so either i need to find someway to dynamically replace <login> with my real destination number (which i already tried but no luck) either something else
inbound calls i've not tested yet
also it feels like there could be some trick with Bridges menu or Destinations/Outbound menu but i have no clue how to use Bridges menu except creating records there
screen below, where 11223344 - is extension i've created for gateway to register on FusionPBX