There are a number of different types of irony, but the relevant one in relation to "The Moment the Gun Went Off" is situational irony. This is a literary technique that gives us the exact opposite of what we'd expected. It's commonly employed in short stories with a twist ending, and, when used properly, it can be very effective.
In Gordimer's short story, situational irony can be observed in the surprise revelation that Lucas, the young Black man killed by the white farmer Marais in a tragic accident, is actually his son. This startling revelation demonstrates that the relationship between the political and the personal isn't always as straightforward as many people assume.
To many people in South Africa, and the outside world as a whole, Marais's killing of Lucas will be seen as yet another act of racial injustice in a country where white racism is the norm.
However, the revelation that Lucas was Marais's son puts a whole different complexion on things. Far from theirs being a relationship based on exploitation, it was actually based on love, the love that a father and son should always have for each other. And that love was displayed quite openly in the moment before that tragic event, the moment before the gun went off.