The following impl_ methods, copied from GridPaneDesignInfo (which is part of the tooling APIs but not part of the runtime), are being added to GridPane in FX 8 as internal, treatAsPriavate methods to support SceneBuilder:

public final int impl_getRowCount()
public final int impl_getColumnCount()
public final Bounds impl_getCellBounds(int columnIndex, int rowIndex)

We should formalize this API in a future version of FX.