When you add a GMask node to the schematic, it is added with an Axis node.
A GMask object (called gmask1, by default), with its parent axis, appears in the schematic. Once you have added the object, you can create the mask. See Creating a Mask.
Adding a GMask node to the schematic will apply the mask to the entire scene. You link the GMask object to specific media to apply a mask selectively. You can also apply a link to exclude a mask.
The selected image is connected to the mask by a blue line with an arrow, and only the selected surfaces are masked.
Excluded images are connected to the mask by an aqua dotted line with an "X", and they are not masked.