How to autostart Gufw with the system?

A short answer is, you really don't need to autostart Gufw. Gufw is just a settings tool for ufw (the real firewall), and they are split separate.

Think of Gufw as a settings dialog. After you do all of the changes and close it, the settings are still in effect (for example, when you setup a new printer - after you close the setup window, your printer setup still stays!).

So there is no need to add Gufw to auto-start, unless you're going to change your firewall settings very frequently. If you enable the firewall and close Gufw, the firewall will still be enabled, even after a reboot.