I am pretty sure it is because the GridContainer arranges its children automatically, so you need to define a minimum size on your RichTextLabel.
it works! thanks very much
fliar | 2017-06-20 01:56
You can also play around with the “Size Flags” to make your label expand itself so if you change the size of the GridContainer dynamically the label will resize itself
Rasmus | 2017-06-20 07:18
Other than normal Label the vertical size flag of a RichTextLabel is set to Shrink Center as the default value - at least on Godot 3. When set to Fill it adapts to the grids size.