You only need gin if you want to index the entire jsonb. For a specific attribute, you can use the default (btree) which I'm guessing is faster.

Yes, as far as indices go, GIN indices are very expensive especially on modification. They're worthwhile in cases where you want to do arbitrary querying on JSON data, but you definitely don't want to overuse them.

If you can get away with a regular index on either a generated column or an expression, then you absolutely should.