If by GUI you mean WYSIWYG, I don’t know of any! Very mysterious to me why this has not been properly taken on given the popularity of markdown.

Once every year or so I check out everything that’s available and try out any new or upgraded packages I can find. All have at least one of the following issues:

• Massive bloat, often electron is significant culprit
• Stuck on the 2 column editor concept, generally with only rudimentary markdown implementation
• Fly by night new projects which are quickly abandoned in beta state
• Only want to access files within a certain subdirectory which may or may not be configurable; this is rarely the only problem but it’s very common in the PKM-type packages

I never quite got it to work properly but Zettlr suits some people. You might be able to cobble something together in Codium. Both those have the bloat issue. There are some self hosted browser-based editors if you are interested in that sort of project. The best and closest I have found is Joplin but it isn’t actually a markdown editor. I wish someone would spin an editor off from its code base; surely the skeleton is there.