To set google mail piping, please go to Support Plus > Add-Ons > Email Piping.
Piping email address should not your site registered users email address.
In google mail setting you have to follow steps and upload client_secret.json file.
You can set pipe account by authentication and it will add for piping.
You can set category for piped email under Assign Category setting. If email receive on that piped email it will create a ticket of selected category.