I have no idea why this should be important for you. But obviously you are mistaken:
Rosa and
Volker Kohlheim (Das große Vornamenlexikon) start their entry: "Old
German fem. forename"
W.Seibicke (Historisches Deutsches Vornamenbuch) lists some 25 examples of
German bearers from the early 18th century on
E. Förstemann (Altdeutsche Personennamen) gives some 15 Reingards (in various forms) from the 9th century on
I don't know whether the name is used much in the Netherlands (it seems to be rather rare in Germany). It is of Germanic origin and definitely older than any "Germany", but it has been used in this country over a number of centuries - so what exactly is your point?