We apologize for the inconvenience.
Synchronization with Google Calendar may have a time lag, and it can take some time for updates to reflect. Please try adding an event to Google Calendar after about a day and check if the synchronization works.
Additionally, if you are using the in-app calendar, it will not sync with Google Calendar.
Please tap the template (or check "Calendar" when entering an event) to confirm if the selected calendar belongs to Gmail.