I don't have first-hand knowledge of Dutch to confirm this, but this usually trustworthy source gives "female":
http://www.meertens.knaw.nl/nvb/naam/is/Geertruij
Google searches for "Geertruij" also only turn up pages talking about "her" and "she" in connection with this name, as far as I can see.
Anyway, the site above gives it as a variant of "Gertrud", and at least in German there is no male variant of Gertrud in sight anywhere :)
This message was edited 4/14/2016, 10:10 PM