I have had a gmail account for 3 years plus.
A few months back, the email just plain quit working regardless of how it was accessed. From the phone or via the web off a PC.
emails sent to my gmail account just simply never arrived. The inbox was empty for most of this year.
I did a lot of stuff trying to get it to work. I even created a couple of new gmail accounts for testing and they both worked. As my internet world sort of revolves around the base gmail account, I hated to delete it. I did that once to try and recreate it. Google will NOT allow that and I lost my favorite account name.
Finally, I decided to just delete the gmail account off of the phone totally, but not the gmail account itself in Google.
I cruised along for a while with just my Yahoo account and things were okay, but Play Store would not work right. I had to download .apk files and that is a pain.
Then I decided add the gmail account back onto the phone. It went thru the normal gyrations we have all seen on the "first startup" and then something wonderful happened. After I got the message "configuring gmail servers", the phone started working with Play Store, gmail was suddenly available on the web. It would receive messages! Then I added gmail as an email account and it works.
and it all syncs up correctly.
long way around the barn yard to suggest that you delete your gmail account off the phone, and redo it again.