After a long time using Gmail, anyone can miss a few emails, hence keeping them in unread status.
In my situation, the Gmail app says I have several unread emails in my Primary inbox on all of my devices. It makes me feel irritated.
We need to list only unread emails in order to check and read them.
You cans search them using is and in search operators.
is:unread in:inbox
is:unread category:primary