Instead of a direct answer, there are a few indirect things there to be told:
1) "String sth" is a property in groovy code that gets changed internally to "private String sth + getSth()/setSth()"
2) "public String sth" is a field in groovy code and it remains unchanged as a field.
3) Stub generators currently generates annotations only for non-private fields and not for properties.
4) The stub generator may start doing something for properties as well - not sure at this point what that will be - whether the annotation will go on to stick to private field or accessors.
5) It is highly unlikely that property "String sth" will later translate to "public String sth" field instead of (private field + accessors).