Using GIN to index a JSON column
GiN(Generalized Inverted Index) is inverted index, a structure that has only one single index entry per a key and store the mapping information(posting list) of all key row to have the same value in the index entry. In postgreSQL, the key is stored in the index entry and mapping information for the key is stored in the posting tree. To search the index entry and posting tree is using a B-Tree. Therefore, GIN is useful when an index must map many values to on row, such as indexing array, documents.

I suppose to that before creating GIN index , you are doing some mistake. as you see here postgresql GIN pg_trgm default operator class 

To Creates a GIN (Generalized Inverted Index) the syntax should be like that

CREATE INDEX name ON table USING gin(column);


Creates a GIN (Generalized Inverted Index)-based index. The column must be of tsvector type.

Default operator class jsonb_ops supports existence operators( ?, ?&amp;, ?| ) and containment operator( @> ). And jsonb_path_ops supports containment operator only. Therefore GIN index is only possible to search for that have a particular key or key-values.

When we use ->> operator of JSONB, PostgreSQL can use B-tree or Hash index for processing the operations. ->> oerator returns the value of the specified attribute in text format. PostgreSQL can use indexes for the text results as compare operands. GIN index can be used by GIN jsonb operator class.

To see the json operator Here

For further ref Here and Here