I'd have to go with file over a GUI, although if the GUI is well done it'd make things a lot easier.
I'll likely end up as the first person to get slapped through the internet for this, but I actually miss XML-based configuration compared to YAML. If it's well designed, and has a well-written XSD file, and you have your IDE setup right, you get good insight into what you're configuring. Granted, I rarely had those all line up right, but when it did it was amazing. Now I'm fighting YAML config for Spring Boot apps, trying to figure out what I'm doing as I go along, and not having the best of luck trying to sort through the docs. (That's probably mostly on me - not a slight on Pivotal; they've got decent docs in place.)