jsonb and comparison operators

Is it possible to get this query (or a similar one) to use an index?

I want to return all rows that have a value of less than 10. I have
arbitrary keys I want to check (not just 'a').

drop table if exists test;

create table test (j jsonb);

insert into test select json_build_object('a', i)::jsonb from
generate_series(1, 100000) i;
create index on test using gin(j);

vacuum analyze test;

select * from test where (j->>'a')::int < 10;

I tried
select * from test where j->'a' < 10::json::jsonb;
but didn't seem to use the index.