Go to settings, applications, click application manager, click the 3 dots menu top right corner, click Special Access, click Optimize battery Usage, click Apps not Optimized, select all apps, scroll down to Gmail and make sure it is not optimized. I had the same issue and Gmail was "optimized", deselected and it syncs as usual now.
the only way i know i have email now is if i go into each account and refresh. yes the account sync is on. it seems it's only for mail. everything else is working properly. i have not downloaded anything in the past few days and this only started 2 days ago.