A G_List can only be written if the type of its elements can be determined, therefore you first have to set the type with "set-type". G_List uses the same types as g_xdr_array, therefore new types can be added with g_xdr_add_type if necessary, see section 5.7.2.
There is no "xdr" method for class GraphicDevice, instead the function g_xdr_graphicdevice has to be used. The reason for this exception is that it is very dangerous to read in GraphicDevice instances with g_xdr_instance, because new GraphicDevice instances could be created or -- even worse -- existing instances could be deleted.
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