Only items created and supported by Google Calendar will sync there directly. Going to Menu -> Calendars -> Display in S Planner will show you which calendars are linked directly to Google. Anything else will only be supported by your Samsung Account. You may have to either create a "Tasks" option in Google Calendar (via web) using that instead or see if there's an app that imports Samsung Tasks into Google Calendar.
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If it ain't broke, use a sledge hammer.