JavaFX: Checkbox labels in a GridPane are not visible

The container is a GridPane, where checkboxes are added with:

gridPane.add(checkbox, columnIndex, rowIndex);

Each checkbox was initialized with a label:

final CheckBox checkbox = new CheckBox(label);
checkbox.setDisable(true);

The result is that there are all crowded. How can I make them more expanded?

The whole thing does have a scrollbar ...