You can use the lookup method. It gets an id and returns a node. But to use it you will need to set an id on the labels.
Example:
public void start(Stage primaryStage) {
try {
GridPane gridPane = new GridPane();
Label a1 = new Label("a1");
a1.setId("a1");
Label a2 = new Label("a2");
a2.setId("a2");
Label b1 = new Label("b1");
b1.setId("b1");
Label b2 = new Label("b2");
b2.setId("b2");
gridPane.add(a1, 0, 0);
gridPane.add(a2, 1, 0);
gridPane.add(b1, 0, 1);
gridPane.add(b2, 1, 1);
((Label)gridPane.lookup("#a2")).setText("texto");
Scene scene = new Scene(gridPane,400,400);
primaryStage.setScene(scene);
primaryStage.show();
} catch(Exception e) {
e.printStackTrace();
}
}