I found it on a list of 'The Most Usual
Christian Names of Men and Women' in the back of a English dictionary from 1796 so
Gertrude was used before the 19th century here.
The full list:
http://www.behindthename.com/bb/arcview.php?id=3206247&board=baby
Plus, the entry for
Gertrude in this database says it was introduced into England in the 15th century and Shakespeare used it.
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Louise x
Is going to Hell
~*~Top Names~*~
Charles Augustus & Isabel Primrose
I bites the heads of little flowers.