The current troubleshooting guide will help you solve the problem when you set your incoming Helpdesk mail settings with Gmail and set correct credentials but still, it returns failure when you get the mails to Helpdesk like in the screenshot below:
The reason for this problem happening is that Gmail has a restriction. To disable it, please follow this link.
Then you will have the following window appearing. You have to click on Less secure app access.
Afterward, this screen will come.
This restriction indeed is preventing you to receive the Gmail emails even though you have already defined the correct credentials. Please it and everything will start working fine afterward. It should be Allow less secure apps: ON.
Then, the successfully received mails will indicate as in the picture below.