i personally feel, you should try to write your class as the main class and include the GUI object as a member variable in it. this way, all you have to do is initialize the GUI in "your class"'s constructor. all you need to do is call show() after the operations in your class have been done with.

this way, if there's any segfaults arising out of your class, you will be able to debug better. just a suggestion!
This is a design question.
No one can say this is better in his hase, than another solution.
You should always fit your tools and solutions to the problem you have, and not stick to one tool/solution/design without knowing what the requirements are.