Unfortunately, the GLT240128 does not have a built-in screen rotation function, however, it does offer a number of features that can help you pull it off, with some work.
First, you'll need to create and upload a font in the correct orientation. Use a mono-spaced font which will allow you to accurately track the cursor location. Then, using the set cursor coordinate command you can print each subsequent character in the right location.
The GLT240128 Touch
appnote uses a simple version of this method in the display class to track cursor position and provide a function to delete characters. If you haven't already, you might check it out for some ideas.