Here is a step by step process below for setting up Mail Flow Monitoring on Gmail:

Login to your MailFlowMonitoring account. Click on Monitors icon in the left bar and then click on New button to create a new monitor.

Fill in the new monitor form that is self explanatory.

Copy the forwarding email provided in the Email section as highlighted below.

Open Gmail.

In the top right, click Settings icon.

click Settings.

Select "Forwarding and POP/IMAP" tab.

Click on "Add a forwarding address" button.

Enter the forwarding email address you copied from MailFlowMonitoring in the popup form and click next button.

Click on Proceed button.

Click on OK button.

Go back to Mail Flow Monitor page in your browser and click on "Inbox here" link given under Email section of new monitor form.

Copy the Confirmation code from the email sent by Google.

In Gmail mail forwarding settings, paste the confirmation code you just copied, in text field against the email address you added in the steps above and press verify button.

Select "Filters and Blocked Addresses" tab.

Click on "Create a new filter" link.

Enter email in From field of the new filter form and click Create Filter button.

In the next section of form check "Forward it to" check box, select the forwarding email address you added in steps above and click on Click Filter button.

Go back to new monitor form of Mail Flow Monitoring and click on "Send a test email" button.

If you receive the success message it means that you've successfully setup forwarding email address and a rule in Gmail.

Save your new monitor by click on Save button at the bottom of the page.

In Monitors listing page of Mail Flow Monitoring you'll see your newly created monitor in verified category. Enable your monitor by selecting on Enable option after clicking on Action button.