Difficult to say without detailed knowledge of the kind of interaction required, the platform, and so on.
There are so many Web and HTML tools out there that a browser front end could well be the simplest if this is server based, but that might not be the solution if you have a single user running a local program on a PC.
If the CLI is 'panel' based, that is like old-fashioned mainframe transaction handling systems like CICS or (sigh) TPMS then it is generally easier to port to a browser front end. But if your CLI is asynchronous vt100 type interaction, where you need to grab keystrokes, then you need the control that a GUI will bring. However I have seen several dismal ports to GUIs. The bottom line is that if it was not designed for a GUI then it may not fit, unless you are really lucky.