It was not Gideon Slabbert (born 1939) who married Clara Bock in 1756 but his father Gideon Slabbert. In fact, Gideon Slabbert junior must have passed away at a young age as he was not listed in his mother's estate. When her estate was inventoried it listed 3 adult children ie Catharina, Huibrecht and Johannes and 4 minors, ie Pieter 15, Neeltje 11, Caspar 8 and Jannetjie 5.
Source: https://www.familysearch.org/ark:/61903/3:1:3Q9M-C9BY-C9Z9-D?i=901&...
Also evident from the muster rolls for the Cape; in 1756 Gideon Slabbert (Slabbert-13) was listed as a single man with 2 sons and 2 daughters. In 1759 he was listed with Clara Bock still with 2 sons and 2 daughters. This corresponds to the children listed in Francina Karsten's estate (MOOC 8/7.45a).
Also see the wikitree profile https://www.wikitree.com/wiki/Slabbert-13 for links to the evidence.