A gateway ancestor? I am pretty sure that I have never seen this term or even a similar one outside of a USA context. And no where else on the American continent, Canada, Mexico, Central and South...or any other place in the world
In the latter areas, there are aboriginal populations mingled with Europeans, the Mestizo. Some of them know their routes back to Spain or Portugal. And then are among those who have come later, who know well from where and who they came, but may or may not want to advertise.
Canada is extremely diverse but overall records at port seem to have been quite accurate and the French Catholic and Huguenot influence has easily allowed sheer uninterupted continuity of records
To my mind, it relates to situations where continuity has been lost in terms of records, relationship, hand down names largely due to a chasm creating migration.
At this point, it becomes the job to to work retroactively back from current to create trees that were never recorded or completely lost in the shuffle. Sometimes this is quite difficult.
Some places had excellent records and contingency plans to preserve it in response to a variety of emergency challenges. Other places strike out on all three and even error rates of the records themselves are higher than normal.
However, if the chasm was not great, records were maintained and simply added in real time. They are just there.
Forgotten again as the accumulation got larger, that some children and their lines might disappear, sometimes to be recalled later as significant people emerge downstream.
It is important to read all these records with this context and all of history and watch with an eye to form criticism as to how it may have been written, where additions have been made, how and why I have actually discovered how this works in the Adels by seeing it from beginning to end.
Additionally, there was an acceptance that birth dates etc were unimportant, it was when you emerged, ie walked on the pages of history and then departed.
Then there is fiction. But to what end. People don't change and always hope that there is something of substance "back there."
Often forgotten is that fiction may be cultivated for the opposite reason. We know how this has worked in modern times.
About 100 years ago, thousands of people plunged over the cliff of their past, took on assumed names and stories simply to save their lives. Some may have kept the flicker but others may have lost it temporarily, or forever.
The past was no different. We can see in some situations where two arms of the same family emerged quickly with different trees with those who stayed being careful not to show connection to a family however noble or of historic interest because of religious persecution which could lead to ruin or death. The one that left might have a different and correct one.
In conclusion, you do your best to establish continuity and context, for which a historical background needs to be part of the collaborative process.
And returning to our premise, I guess my dad was a gateway ancestor for his paternal pedigree as I am the first generation of that line to be born in North America - representing his Eastern and Western European and British and Irish gentry and nobility.
And what couple brings that all together - surprisingly Yaroslov the Wise and Ingegard Olafstdottor - perhaps they are the most pervasive Adam and Eve of the Northern Hemisphere
I imagine in this generation that I am in the majority. And then that will change the nature of the discussion all over again