Missing 'Notes' folder in Gmail on iPad and iPhone

On both my iPad and iPhone devices I have the default mail app, in settings with the mail app i choose gmail as my default or primary email account, within the mail app that I choose my gmail account to be, there was a folder called notes that I could have created or the folder could have been a default folder, At times while using either one of the devices, I've sent emails to myself which they'll come to my inbox and then I'll move them to the notes folder. The folders on both devices has disappeared or I could have by mistake deleted them while using one of the devices, I can't find the notes folder on either device, how can I retrieve the notes folder ?

[Re-Titled by Moderator]