Re lack of evidence of registration of birth for Qld BDM ..... I have already discussed this, but please, please, remember that where a registration was not made in person at the Registrar General's HEAD Office, that there's always the possibility that if made at a local office that it was not ever forwarded to the Head Office. May I just mention that I have no real physical experience of researching births/baptisms in records in Qld, but that I have such real physical experiences of researching births/baptisms in records in NSW, including back long before centralised, computerised registration processes. May I also mention that I have several living ancestors who spent a lifetime career in NSW BDM, including at Senior Levels, involved in the development of the EDP system. That does not mean I am right or wrong, but ...... I can assure you that if Qld BDM administrative procedures of the 1860s and 1870s followed NSW admin procedures of that time, that it is not at all unusual to NOT find any number of birth registrations from any particular locality. I share (and I have previously mentioned this at RChat) : Together with a church archivist I found around 800 bdm events in their NSW parish register that were NOT listed on any of the NSW BDM indexes (not on the fische, nor the CDs, nor the online index). That's from one locality. It took years and years for NSW BDM to verify this, and then to accept and then to update their records. NSW, like QLD has many rivers that can flood. Once civil registration commenced for BDM, many of the areas (including Ipswich in Qld) had the registration process for BDMs as a part time function of the local (Petty Sessions etc) court clerks. In NSW (and perhaps Qld) there's many of those court records that were lost in floods, fire, or other natural disasters, just as there's perhaps clerks who failed to forward quarterly summaries to Head Office.
Just because today's BDM officers have not found a civil registration for a particular person does NOT mean that such a registration did NOT take place. It may well be that neither Gordon nor Julia registered the births of their sons (I think you may find they could have attended ANY deputy registrar anywhere in Qld, not just the one nearest their locality).
If Gordon's deceased estate was probated, does that file include information about his sons? If they are named as his sons, under his will, then any question as to their being his legitimate sons or otherwise is surely overcome by the proving of that will in accord with Qld laws.
JM