Irrespective of technology, at some point the backend, C in this case, has to speak to the front end through some middle layer. Hopefully the middle layer and the front end will be self contained for the sake of modularity, separation of concerns, and avoiding the big ball of mud.

Charged with the task of making a GUI, I'd start with HTML, CSS, Javascript and REST. Absent some compelling reason to pursue another course, that's where I'd end.

Good luck.