> Qucs is a GPL circuit simulator. And if you want the GUI option, you might want to try out QucsStudio, which uses Qucs under the hood, and is free to use, but binary-only.

Huh? I used Qucs many moons ago and it definitely was complete with GUI.

https://qucs.sourceforge.net/screenshots.html

Unfortunately Qucs is unmaintained, probably doesn't compile anymore on newer distributions and its binaries were therefore removed from many repositories. Another problem: Qucs was released as GPL while QucsStudio, which uses its sources, has been kept closed by the authors; I smell a GPL violation here.