They are categorized in the GDL Manual (which you have probably found already), so if you have a good guess, you might be able to find the right one.
You can try to use the “Dump library parts of selected elem…” command from the Library developer menu (palce a random object, select it, then use the command), the result report (check the report window) lists all plan global names and values along with the object’s parameters.
About accessing the Library Developer menu, see here.
Dominika Bobály
Product Manager
GRAPHISOFT SE