Hi Pascal,
in principal, the GUI should be separated from the underlying system which controls a certain machine or reads from a certain device.
The binding between the GUI application and the underlying system is implemented within the Device Class / Device Driver. From the GUI perspective, the Device Class is the abstraction of the underlying machine (e.g. to start/stop an engine, or to read the current speed). The Device Driver is the API to control the machine.
As a result, the two parts can be implemented separately: The GUI application can be developed without having the real machine and the machine control logic can be implemented without having a GUI.
O.k. this means: Please try first to implement the machine control (e.g. by setting the GPIOs) and then implement the binding between the GUI and the machine.
The provided example 'DeviceIntegration' demonstrates this binding. According to your last post, the question is how to implement this machine control logic.
For that purpose, please have a look into the STM32CubeF7 embedded software - there you will find several examples that accesses the GPIOs.
Best regards,
Manfred.