Don't build a GUI if you don't have the skills to build a GUI. You won't be able to compete on features in the long run, and you certainly won't be able to compete on design in the short run. And certainly not without devoting 100% of your time towards it.
Instead realize that there are three primary types of interfaces that are in wide use today: GUI, API, and CLI.
If you're a backend dev the last two are much easier for you to build. So instead of building a GUI, build and API. Hire a designer to design you a basic 1 page marketing site. Keep it simple. Put up API docs on http://readme.io/ or something like that. Then iterate on features. Once you solve someones problem well enough find a second person. Iterate until finding leads is your primary problem, not closing them. At that point switch to marketing fulltime.