I am not sure how to make the gui, but I know how to do text-to-speech. You can use espeak:
sudo apt-get install espeak
espeak 'Hello world'
By default, espeak's voice is hard to understand (for me). To find a voice, use:
sudo apt-cache search espeak
Then, find a voice. Install it. Then, run
man espeak
to find how to change the voice. It should be simple.
Here is an example of something I made using espeak: https://github.com/Merlin04/robotthingy
Edit: I think you could make a GUI with Kivy. It uses python.