Hi,
I read your project description. I have worked several projects using Raspberry pi/Arduino. I have completed Raspberry pi based hardware control (relays, temp sensor, vending machine control, RFID reader, arduino board etc) that are connected to pi via GPIO and usb/serial ports. Let me know more detail about t his task. I assume you want to monitor the GPIO ports and send the status to the central location (web server) via a GSM modem.
The GUI design depends on how you are going to use the system. If the PI is used as remote console to monitor with out any user interaction then we can have a minimal UI information. If the PI will be used with a display for user interaction then the GUI should be form based. I cannot disclose more information prior starting the project. I have created a several applications with lot of user interaction. One such example is PI with a touch screen for a swimming pool paying machine. This app consist of UI where user can navigate through and at the end it is having a form that will be filled by the user.
Thanks!