The central idea of gordimer’s essay “1959: what is apartheid? ” is that

To understand better the central idea, let's first define Apartheid.
Apartheid is the segregation of Whites and Black. Where Blacks, mainly in South Africa, are not given equal opportunity with the Whites. 

The central idea, therefore, of her essay is the detrimental impact of racism on the social reality and relationships of the two races.

She exposed the bad human relationship, and how the Blacks were discriminated in terms of employment, education, and ownership of properties.