Currently only labels and zone stamps can read the properties of their parent objects. Many times it would be useful for any GDL object to see their own properties.
In many cases older objects have parameters for values that would be better expressed as properties, but migrating to using properties is hindered by the fact that an object can't see them. For example, a door's fire rating and similar qualities are more natural to express as a property. They should be accessible from the object to show the user door parameters that are relevant to those qualities.
Most recently I wished for this functionality when converting the properties of a landscaping element standard (MaisemaBIM) into a property set. This information is useless to the corresponding plant objects, and the user would have to set the information twice, again as parameters, to have the object look right.
Along the years many others have asked about this at the GDL forum:
https://community.graphisoft.com/t5/GDL/GDL-How-can-I-get-user-defined-properties/m-p/305160
https://community.graphisoft.com/t5/GDL/Linking-properties-into-GDL-Parameters/m-p/323998
https://community.graphisoft.com/t5/GDL/Properties-in-non-label-objects/m-p/265703
https://community.graphisoft.com/t5/GDL/Defining-values-of-ArchiCAD-propoerties-through-a-GDL-script...
The last thread even links to an add-on that lets the user query properties.
(In some cases it would be also useful to write properties, but this is another matter and probably makes less sense in the big picture.)
Open