These are the most common causes of this problem:
Network connectivity problems. To fix this, check your network connectivity to make sure that the connection is working.
Computer time settings. The local time in your computer should be set in standard time, synchronized with an Internet time server, and the timezone set correctly. As a security measure, we are checking the time stamp to verify upload status and prevent unauthorized uploads. The time difference must be less than 15 minutes. To fix this,
Change time settings in your computer. You may refer to these articles to change time settings for Windows 10 or for MAC OS X.
Refresh the Gmail page and see if Gmail loads completely.