I currently use TinyWall Firewall, it works very well, it’s small/portable, no complaints I even donated to the Dev but I would really prefer open source, also it needs to be user friendly like TinyWall so my non-tech family members can/will use it like they do with TinyWall.
There seems to be a misunderstanding:
One of such GUIs, is TinyWall, that is also FOSS (GPLv3). I see people have suggested some more.
To be precise, all these options are inferior in functionality to firewalls like ZoneAlarm… but since you’re asking for a non-tech friendly solution, they should be adequate.