Go into Windows 10 Mail app Settings (gear icon at bottom of Inbox), choose Manage Accounts. There should be an error there to refresh sign in for Gmail so do that first.
Then go into Google settings to make sure Less Secure Apps are allowed:
https://support.google.com/accounts/answer/6010...
This should put the account back into operation. If not go into Mail Settings > Manage Account to delete the Gmail Account, restart PC, go back to the same place to Add it again. Make sure the Advanced Sync Settings are all correct including how far back you want your mail folders downloaded to the Mail app for Gmail.
If that doesn't fix it then see these fixes for Gmail in Mail app:
https://www.hubcustomersupport.com/blog/how-to-...
https://www.blakbin.com/2019/06/windows10-error...
https://superuser.com/questions/951270/how-to-s...
https://superuser.com/questions/1461757/0x80070...
One of those usually does it.
I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I solved your problem, I will keep working with you until it's resolved.
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