Try following this link from the Help Center for help on syncing your Garmin watch to your Grokker account: https://support.grokker.com/hc/en-us/articles/360000423472-How-do-I-connect-my-Fitbit-or-Garmin-account-with-Grokker-
Just some background information, Grokker will only sync the following activities from your Garmin watch: walk, run, bike, swim, hike, elliptical, and workout. If you are still having trouble connecting your device, you should try reaching out to customer support at support@grokker.com and they should be able to help you out more!