We're talking software here, not boxes.  It seems fairly clear to me that the usual rule for signalling interworking applies: it has to be done somewhere, and the choice of where depends on the particular context.  Back to my first point: in the first case, you have to add SIP<=>H.323 interworking software at one or more GW.  In the second case, the GK will interact with SIP second-line components (registration servers, redirect servers, etc.) via TRIP or Annex G, depending whose border elements speak the other network's language.  Call signalling interworking still has to happen at the GW level, by definition.