A GEOPOSITION Structure claims :ISSERIALIZABLE is true, meaning it can be used with READJSON() and WRITEJSON().
But this is false, as demonstrated below:

(Typed at the terminal interactive prompt)

set x to latlng(90,90).
print x:isserializable.
True
writejson(x, "some_file").  // allows you to write it
set y to readjson("some_file").   // but does not allow you to read it back.
Method not found: 'Default constructor not found.. ctor() of kOS.Suffixes.BodyTarget'.
at Interpreter line 7
set y to readjson("some_file").
         ^

For objects to be serializable, all the objects inside them that they save need to be serializable too. BodyTarget, which is saved as part of a GeoCoordinates, isn't serializable because it has no default constructor. (All serializable items need a default constructor because the way SafeSerializationMge.CreateInstance() works, it makes a dummy of the object first, then populates its fields from what's in the file. If it can't make the initial dummy of the object (no default constructor), then it can't do the work.)