I'm curious what is the reason for a GUI for such a simple utility? The command line for this kind of tool seems like the perfect interface. Just copy a URL, paste it, press enter. No buttons to press, etc.
Unless you have reason to want something other than the default format that it will pick (perhaps for compatibility with an old set-top media box), or need other options like turning off SSL verification for some sites (it does much more than youtube) that have this misconfigured, options for playlist, ..., ...
For the simplest and most common use case it is just copy+paste, but for others there are a lot of options. I prefer the CLI and manpage myself, but some are far more comfortable with a GUI, finding the options more discoverable that way.
(though having had a quick look at the project page it looks like this doesn't offer options beyond format selection, but for some users that is useful enough to warrant a GUI)