My Gear Fit 2 Pro doesn't synchronize changes from my iPhone 7 Calendar to the Gear. At first time connecting phone and Gear the calendar will be synchronized for about the next two weeks. After this, there are no changes at the calender seen at the Gear - after the first time synchronized date the calender at the Gear keeps empty. Doing a reset of the pairing, same procedure works again: about two weeks are synchronized during the first connection - neiter any change nor any future calendar entrys will be added. the iPhone-Calender itself is synchronized with an Exchange Mail Server.
You can try one thing. Go to iphone settings, scroll down to galaxy watch and toggle on calendars. Again go to phone settings, scroll down to calendar and sync all events.