The javadoc for executeInsert(String) and (String, List) show the result as:
A list of the auto-generated column values for each inserted row
For executeInsert(GString gstring):
"return: A list of column values representing each row's auto-generated keys"
I think it's actually the same as the two previous one, it doesn't only return the keys but any generated value, more like it's stated in the comment: "returns the values of any auto-generated colums". Please note that "colum" is mispelled.