Mail still constantly asks for Gmail password

I've had this 2015 MacBook for three years and have used the same gmail account with this MacBook for three years. About one or two days ago Mail constantly asks for gmail's password. I've tried entering a few times - it might work briefly, but reverts back to the same sign-in issue. Nothing change at my end - so it's either Google or Apple. Anyway, hope this resolves itself on its own.

Anyone else having this problem?

David