Paton is using the laughter of Gertrude and Absalom's girlfriend to symbolize the kind of lifestyle of which these two women had been a part.  It represents their loose morals and lifestyle.  When Gertrude goes to stay at the mission house with her brother, Stephen, she is not willing to change her lifestyle, and this unwillingness to change is shown by her careless laughter.  Absalom's girlfriend on the other hand does want to change, and when she is confronted about her laughter, she apologizes and changes.  This true change is evident in her moving to N'dotsheni with Stephen and his wife; whereas, Gertrude's unwillingness to change is shown in her abandoning her son and brother and returning to her life in the slums.