Start GCal when you sign in
Open Settings → General and enable Launch at Login. macOS may list GCal under Login Items. If launch is blocked, review that system setting and reopen GCal.
Choose what appears at launch
Enable Hide window at launch when you want the menu bar, notifications, widgets, and other native features ready without bringing the calendar window forward. Turn it off when you want the main window to appear at startup.
Choose Dock and menu-bar visibility
The Dock and menu-bar icons have separate controls. Keep at least the access points you want enabled. Closing the calendar window does not necessarily quit GCal; choose Quit GCal when you want to stop the application.
Free memory after closing the window
Free memory when the window closes releases the hosted calendar session after the main window closes. Reopening the window may therefore take longer and may need to reload Google Calendar. Leave it off for faster reopening, or enable it when reducing idle memory matters more.