This button has worked for me in the past, but now seems to do nothing. I do not see the message indicating that it is re-syncing the data. Noting is removed from the watch and re-added. I updated on 5/17 to the latest phone and watch os, 9.3.2 and 2.2.1. I think my sync broke then.
An earlier reply indicated that their problem may have been caused by their declining an event that was previously accepted getting stuck on their watch. I think that may be what was also breaking my sync. I had an even in my gmail calendar that I had accepted, but then declined on the browser. It was stuck on my watch. I just now declined it from the watch calendar app and it is gone.
Think my calendar is correct now, but the "Reset Sync Data" button seems to do nothing still.