Not able to see the related records when I open an email in gmail

Hi @Kandula Hemanth Kumar

Your screenshots narrow this down. I think the problem is that you're using the wrong Salesforce panel, not a broken configuration. 

What the screenshots show 

• Your pane config is fine. The Email Application Pane Default has Related and Tasks tabs with Email Templates, Log Email or Event, People and Contacts. It's the default page and it's assigned to System Administrator. This rules out the "empty pane" theory.
• Cookies and clickjack aren't the problem. Third-party cookies are allowed, and the clickjack settings look normal. Your Incognito test also proved nothing, because Chrome blocks third-party cookies in Incognito, as your own screenshot says.
• The panel in your first screenshot is Chrome's side panel, not the Gmail add-on. It has the Salesforce logo and pin/close buttons at the far right, and your last screenshot shows the same panel. That's the Salesforce Chrome extension, whose home view shows Contextual Insights, My Goals and Search. It works on non-email pages because it's authenticated, as you noticed. It isn't what renders the Email Application Pane, so it stays blank when you open an email.
• The Gmail add-on isn't installed or pinned. In your first screenshot, Gmail's own right sidebar shows only Calendar, Keep, Tasks, Contacts and the "+" button. There's no Salesforce icon. The Email Application Pane (Related / Tasks tabs) appears there, not in the Chrome side panel.

This is an inference from the screenshots, so verify it with the steps below. 

1. Close the Chrome side panel. It's a distraction for this test.
2. In Gmail, click the + in the right sidebar (Get add-ons) and install the Salesforce add-on from the Google Workspace Marketplace. Grant the permissions.
3. Open an email and click the new Salesforce icon in the right sidebar.
4. When the add-on asks you to log in, use the custom domain / sandbox option with your sandbox My Domain URL. Don't use the default production login.
5. Test with an email from an address that exists on a Contact or Lead. Your test email is self-sent, so you may see an empty Related tab even once the pane works.

Other notes

• Cookie exceptions: add [*.]salesforce.com alongside your existing [*.]force.com and [*.]sandbox.my.salesforce.com entries.
• Clickjack setting: you unchecked "headers disabled" only as a test. It didn't help, so restore whatever it was before. I can't tell from the screenshot what the original value was.
• Chrome flags: I'd skip the chrome://flags suggestion from the thread. It isn't backed by your evidence.

One question: is the Gmail account personal (@

) or Google Workspace? I haven't verified that personal accounts are fully supported for the Gmail add-on, and the "Upgrade" button in your first screenshot suggests personal. If the add-on won't install or authorize, that's the first thing I'd check in Salesforce Help. Also confirm in Setup > Gmail Integration and Sync that the integration toggle is on and the user has the required permission set. I haven't verified the exact permission set name. 

I hope you find the above information helpful. If it does, please mark it as Best Answer to help others too.