Gideon van Zyl's death notice here: https://familysearch.org/ark:/61903/3:1:3Q9M-CSQJ-6ZD1?i=1885&c... clearly states that he was still married to Elizabeth Margaretha van Zyl at the time of his death on 27 Jul 1836. The inscription reads "Married to Elizabeth Beukman, surviving".
This error comes all the way from CC de Viliers' Oude Kaapsche Familien and persists in all records I could find. To make the "second marriage" to Regina Barbara Barbata Wolfaardt, b5c1 fit, Elisabeth Beukman's death date is often given as "before 29 Dec 1804", which was Regina Barbara Wolfaardt's marriage date to a Gideon van Zijl. However, Elisabeth Beukman according to http://www.moustache.nu/genea/parentelen/z-afr-b5/z-afr-b5.htm#BM15238 passed away on 09 Jul 1840, which fact is supported by an inventory filed for her estate in 1840 : DEPOT KAB SOURCE MOOC TYPE LEER VOLUME_NO 7/1/159 SYSTEM 01 REFERENCE 77 PART 1 DESCRIPTION BEUKMAN, ELIESABETTA. WIFE OF GIEDEJON VAN ZIJL. INVENTORY. STARTING 18400000 ENDING 18400000 REMARKS FILED 1840
The likely candidate husband for Regina Barbara Wolfaardt is this Gideon van Zyl, b5c4d2 whose wife Maria Elisabeth van Zyl passed away before 25 Nov 1803 according to her estate inventory : http://databases.tanap.net/mooc/main_article.cfm?id=MOOC8%2F55%2E28a
Regards,
Dirk