The phrase "Gateway Ancestor" could be confusing because it has specific meanings to specific projects. For instance, in the Magna Carta Project, it refers to a verified descendant of a Magna Carta surety who immigrated to North America and is included in Douglas Richardson's Magna Carta Ancestry and its sequel, Royal Ancestry.
This has led to discussions about Magna Carta surety descendants who emigrated elsewhere, for instance Australia or New Zealand, but there has been no recognized work identifying and documenting such persons.
I am not aware, either, of any parallel work documenting overseas descendants of Welsh royalty. My ancestor Sampson Waring crossed the sea from Shrewsbury, England to Virginia and Maryland; several generations earlier his ancestors were descendants of Welsh nobility, and given the penchant of Welsh nobility for honoring long pedigrees, Sampson Waring is clearly the kind of "gateway" you describe.
It's just that there is no organized way of calling him that, in distinction from people who aren't called that! There would need to be a definition of gateway from where? gateway to where? and gateway recognized by whom?.