Gmail has been acting up weird from the past one week. The app failed to sync, automatically and manually, at least for me. After searching for a while, I found out, thanks to Android Police, that I was not the only one facing this issue, there are many others like me.
There’s a big thread on the Google Support forums where users are complaining about the loss of sync on Gmail, and even Google is aware of the issue. One of the employee mentioned that Google is investigating the problem, and are now looking for a solution to fix this.