Error 'mydomain.my.salesforce.com refused to connect' in Gmail Integration Chrome extension

1. To improve the performance by reducing the data processing and fetching Clear the cache of Gmail Integration extension.

2. Browsers retain a lot of information over time, and while most browsers are configured to clear things like cookies and cache periodically, a build up of these can cause issues with logging into websites and page rendering. Follow this Google help site to clear the cache and cookies in Chrome.

3. Adding Trusted Sites:

• Click Chrome settings |  On that page go to Site Settings and open it up 
• Under Permissions find Cookies and site data 
• Locate 'Allow' section and click on the 'Add' button 
• Add the following three trusted sites, just cut/paste each line in one at a time:

[*.]content.force.com
[*.]salesforce.com
[*.]Salesforceiq.com

4. Removing the Gmail connection from Third-Party Account Links:

• In Salesforce | Go to Setup > Users and select the affected user
• Navigate to 'Third-Party Account Links'
• Click on 'Revoke' for Google

       Note: there are maybe some third party connections that still have the old legacy account connected (e.g Gmail Integration with Inbox) , Revoke / Delete if there are any or check the user if that email address has been connected with the other user before)

5. In Salesforce Session settings following options should be checked under Require secure connections (HTTPS - The should be checked by default after the Summer '21 release):

• Require secure connections (HTTPS)
• Require secure connections (HTTPS) for all third-party domains​​​

6. Try reinstalling the Salesforce Gmail Integration chrome extension.