It appears that turning off Power Saving fixes the issue. Bought my S7 the end of September 2016. Immediately had sync problems in 3 Google apps: calendar, contacts, and gmail. Sometimes 2 of 3 apps would sync after 3 weeks of waiting. Especially frustrating was not being able to get appt. notifications for as long as 3 weeks after they were made and sometimes after I was supposed to be there. Didn't contact tech support but did check online. However, none of the proposed solutions fixed the problem. I slowly worked my way thru every "Accounts" setting on the phone for each of the 3 Applications. When I worked my way thru each of the phone "Accounts" settings, I found that the second I switched power saving off, all Google apps immediately synchronized. Since that time, new appts, contacts, and mail additions and changes immediately appear in the corresponding apps on my desktop and tablet. (I was only using the medium, not high, Power Saving level.)
Can't believe that this is a problem that Samsung isn't taking more seriously.