It depends on what GUI toolkit you want to use, but in general there are 3 ways.

The major consideration, is 'not blocking the gui', otherwise known as 'keeping the event loop going'. In gui's, the event-loop is the main program which is running, so if you need to run another process, it's either "fork-and-exec", threads, or finding a way to constantly call "update" or "do-one-event-loop" .

You really need to be more specific in your question, because there are MANY different ways to do it, and it all depends on the GUI toolkit, and the program you need to spawn( and whether you need realtime results from the spawned program).