Let say that I create a table named people with a computed field named data that the function is that:

create or replace function people_data(person people)
returns text as $$
  select 'blah'
$$ language sql stable;

If I try to perform an insert returning that field, the query will fail:

mutation {
  insert_people(objects:{name:"John Coltrane"}) {
    returning {
      id
      name
      data
    }
  }
}