Some of them expressly does like Olivier Dentan who say in this site "Open GDL code", but He says also "Ne modifiez ou désactivez ces scripts que si vous avez une bonne connaissance du GDL. "
The danger is more important for the unexperiment user than for the writer. The objects are more and more complex and the consequences of modifications are often not very easy to see for the unexperiment user.
The advantage of the open code is the possibility of minor adaptation of an object to a particular purpose by the "medium-experiment user".