PostgreSQL GIN indexes on arrays

I'm working on an app to display content shared by users and I'm using PostgreSQL Arrays to handle the security model.

We support public and private content and I have two queries that I need to optimize. From PostgreSQL documentation I need to use GIN indexes when indexing array columns but I can't get PostgreSQL to pick them.

Here is my data and indexes definition:

-- Table: public.usershares

-- DROP TABLE public.usershares;

CREATE TABLE public.usershares
(
  id integer NOT NULL,
  title text,
  sharedcontent text,
  shared_on timestamp without time zone,
  roles text[],
  claims text[],
  users integer[],
  private boolean,
  CONSTRAINT pk_id PRIMARY KEY (id)
)
WITH (
  OIDS=FALSE
);
ALTER TABLE public.usershares
  OWNER TO postgres;

-- Index: public.idx_arrays_private

-- DROP INDEX public.idx_arrays_private;

CREATE INDEX idx_arrays_private
  ON public.usershares
  USING gin
  (roles COLLATE pg_catalog."default", claims COLLATE pg_catalog."default", users)
  WHERE private = true AND claims <> '{}'::text[];

-- Index: public.idx_arrays_public

-- DROP INDEX public.idx_arrays_public;

CREATE INDEX idx_arrays_public
  ON public.usershares
  USING gin
  (roles COLLATE pg_catalog."default", users)
  WHERE private = false AND claims = '{}'::text[];

-- Index: public.idx_sharedon

-- DROP INDEX public.idx_sharedon;

CREATE INDEX idx_sharedon
  ON public.usershares
  USING btree
  (shared_on DESC);

-- Index: public.idx_sharedon_notprivate

-- DROP INDEX public.idx_sharedon_notprivate;

CREATE INDEX idx_sharedon_notprivate
  ON public.usershares
  USING btree
  (shared_on DESC)
  WHERE private = false;

-- Index: public.idx_sharedon_private

-- DROP INDEX public.idx_sharedon_private;

CREATE INDEX idx_sharedon_private
  ON public.usershares
  USING btree
  (shared_on DESC)
  WHERE private = true;

QUERY #1: Although I have a partial GIN index for this query 'idx_arrays_private', PostgreSQL uses the 'idx_sharedon_private' (which is also a partial index but that doesn't include the array columns (roles, claims and users)

select *
from   usershares 
where  
    ( usershares.private = true 
         and usershares.claims != '{}' 
         and ( ( array['adminX'] && usershares.roles /* to see private content user has to belong to one of the roles */ 
                                and array['managerX'] <@ usershares.claims ) /* and have all the required claims */ 
                or array[]::integer[] && usershares.users /* or just be member of the list of authorized users */ ) ) 
order by shared_on desc
limit 100
offset 100;

QUERY #2: Although I also have a partial GIN index for this query 'idx_arrays_public', PostgreSQL uses the 'idx_sharedon_notprivate' (which is also a partial index but that doesn't include the array columns (roles, claims and users)

select *
from   usershares 
where  
        ( usershares.private = false 
             and usershares.claims = '{}' 
             and ( array['admin'] && usershares.roles /* to see public content user has to belong to one of the roles  */ 
                                   or array[1,2,3,4,5] && usershares.users /* or be a member of the list of authorized users */
                  ) ) 
order by shared_on desc
limit 100
offset 100;

Script to generate test data:

TRUNCATE TABLE usershares; 

INSERT INTO usershares 
            (id, 
             title, 
             sharedcontent, 
             shared_on, 
             roles, 
             claims, 
             users, 
             private) 
SELECT x.id, 
       'title #' 
       || x.id, 
       'content #' 
       || x.id, 
       Now(), 
       array['admin','registered'], 
       '{}', 
       '{}', 
       false 
FROM   Generate_series(1, 500000) AS x(id); 

INSERT INTO usershares 
            (id, 
             title, 
             sharedcontent, 
             shared_on, 
             roles, 
             claims, 
             users, 
             private) 
SELECT x.id, 
       'title #' 
       || x.id, 
       'content #' 
       || x.id, 
       Now(), 
       array['admin','registered'], 
       array['manager', 'director'], 
       array[1,2,3,4,5,6,7,8,9,10], 
       true 
FROM   Generate_series(500001, 750000) AS x(id); 

INSERT INTO usershares 
            (id, 
             title, 
             sharedcontent, 
             shared_on, 
             roles, 
             claims, 
             users, 
             private) 
SELECT x.id, 
       'title #' 
       || x.id, 
       'content #' 
       || x.id, 
       Now(), 
       array['adminX','registeredX'], 
       array['managerX', 'directorX'], 
       '{}', 
       true 
FROM   Generate_series(750001, 1000000) AS x(id);