A slider in GUI to control PWM values?

How would I go about to create a GUI Slider/trackbar that changes the PWM data on the fly?

Would I have to write it in another language and then communicate with the arduino over COM1?

Assuming the GUI in question is running on a PC, and the Arduino is connected to the PC via USB, you could implement the GUI in Processing or whatever other framework you preferred and send serial commands to the Arduino, and write a sketch for the Arduino which receives the serial commands and executes the corresponding action. If you don't need the sketch to provide any other logic apart from obeying commands from the PC, you could run Firmata on the Arduino and use a Firmata client on the PC to provide the GUI.

I took a look into the processing GUI and it looks simple enough to just send the data over com 1 to the Arduino sketch.

This can be done on an Arduino with a TFT LCD and Touch Screen - is this what you were looking to do ?