You may need to purchase a Gathering Attachment
Part No. 200 217 101
BUT be sure it is suitable for your model first
(I am unsure which one you have)
Thread tensions are set at manufacture to all = 3 for standard work, your manual is generally helpful for many answers.
The "Janome Overlocker Sewing Guide" is a very detailed publication and has this to say about Gathering without any additional attachment.