It is not currently possible to specify an Insets in FXML without a custom builder. Adding a static valueOf(String) method to the Insets class would allow a caller to specify insets using a string-based attribute value. For example (assuming that valueOf() converts a string of the form "top, left, bottom, right" to an Insets):

<GridPane hgap="4" vgap="6" padding="4, 8, 4, 8">