I want to recommend if you are actually accessing your Gmail account then you can take a backup of Gmail messages using Google takeout. After that, you can open it in any MBOX-based email client. On other hand, install any email client (like Mozilla Thunderbird) and set up an account with Gmail...
When I was also suffering from this problem I used Malwarebytes to fix the issue with my PC and now it works well. So I would like to suggest that try this method first then go ahead.