Before following the below steps, you will need to make sure your Gazelle calendar has been exported to your Google calendar. You can find those instructions HERE
Adding Google Calendar Notifications
On a desktop, click the dropdown arrow next to your Gazelle calendar. Select "Edit Notifications" from the menu.
Click to "Add a Notification" for events and choose whether you would like to receive email notification or push notifications (which should push to your mobile app). After setting the time frame for reminders, hit save.
You are all set!