IMO, a GenTran panel isn’t the same as a standard subpanel. It is just extending the circuits that at are supplied from the existing panel and as a generator transfer switch/panel, is listed for the application.
The subpanel in the OP’s post is actually supplying the two circuits that originate from within it with the neutrals and grounding conductors being connected to a bus in a different panel.
Does it make any difference if the this subpanel is adjacent to the main panel, or across the room?
Why would the installation requirements and circuit connections be any different in this case?
