On the top module in GT Player Express, click the Edit button. Under Chain Input, "Stereo" is the default setting, assuming that a Stereo signal (using Input 1 AND 2 of your audio interface) is sent into the software. Since your guitar is only connected to either Input 1 (left) OR Input 2 (right) of your audio interface, you have to click on the corresponding button (left or right) for the software to know where the signal is coming from and to pan it to the center of the stereo panorama.