PostgreSQL: pg_trgm full-text search with jsonb columns?

Question

I have a table with a jsonb column where I store variable data. I would like to search this column and also find fragments (leading or trailing whitespace). I think I know how to do this with text columns but cannot wrap my head around how to achieve this with jsonb columns.

There are two scenarios that I would like to achieve:

1. Search a specific key inside the jsonb column only (for example data->>company)
2. Search the whole jsonb column

For text columns I generate gin indexes using pg_trgm.

Install extension pg_trgm:

CREATE extension if not exists pg_trgm;

Create table & index:

CREATE TABLE tbl (
    col_text  text,
    col_json  jsonb
);

CREATE INDEX table_col_trgm_idx ON tbl USING gin (col_text gin_trgm_ops);

Example query:

SELECT * FROM tbl WHERE col_text LIKE '%foo%';   -- leading wildcard
SELECT * FROM tbl WHERE col_text ILIKE '%foo%';  -- works case insensitive as well

Trying the same with the jsonb column fails. If I try to index the whole column

CREATE INDEX table_col_trgm_idx ON tbl USING gin (col_json gin_trgm_ops);

I get the error

ERROR (datatype_mismatch): operator class "gin_trgm_ops" does not accept data type jsonb

(Which makes sense). If I try to index just one key of the jsonb column I also receive an error:

CREATE INDEX table_col_trgm_idx ON tbl USING gin (col_json->>company gin_trgm_ops);

Error:

ERROR (syntax_error): syntax error at or near "->>"

I used this answer by @erwin-brandstetter as a reference. Any help is highly appreciated (and no, I don't want to implement Elasticsearch as of now :) ).

---

Edit: Creating the index like this actually works:

CREATE INDEX table_col_trgm_idx ON tbl USING gin ((col_json->>'company') gin_trgm_ops);

And querying it also doesn't lead to an error:

SELECT * FROM tbl WHERE col_json->>'company' LIKE '%foo%'; 

But the result is always empty.