How to make my Drupal custome Service return data on GET?
The short answer is you can't, unfortunately (AFAIK). For all non action resources, the Services module assumes that they are part of the entity C.R.U.D. spectrum, and then decides what to do based on the HTTP method. For example:

POST = Create
GET = Retrieve
PUT = Update
DELETE = Delete


So unless you're building an entity C.R.U.D. layer, you'll have to use POST on the actions array. Alternatively you could use hook_menu() to make your own custom page_callback that outputs JSON, then it could be accessed over GET. Another alternative is to use something like Views Datasource and compose a Views JSON to output your data (also available via GET).