The GTT supports only the control characters which are listed in the manual; 7, 10, and 13 decimal.
However, if for instance you are building a keypad to which you'd like to add a backspace key, you can create that functionality with a little work. You can find an example in the GLT240128 Touch Screen
appnote, take a look at the DeleteChar() function in the Display.cs file. Essentially, you'll have to use a mono-spaced font, keep track of the cursor position, and finally draw a rectangle over the last character and move the cursor back to execute a backspace operation.