Hi Gerhardus
The info on your tree re William Gardner is incorrect. The William James Gardner currently in the tree was not Johanna Magdalena's William - who had no second name as far as I could find.
According to the document KAB CO 4038 Ref 2 (A request for military pension). William died 20 Mar 1835 and left Johanna Magdalena destitute.
The date of the Gardner x Oosthuizen marriage in the same document is 24 Feb 1817.