I am not sure what light texture you are looking for, a screenshot might be needed if I don't answer your question.
If you are looking for the actual image file, take a look at sg_sfx.
If you are looking for the shader script, look in sg.shader.
Any shader you find in GTK Radiant, the image file should be placed in the identical folder. There are a few exceptions (the web shaders I created for NNC), but these exceptions are most likely mistakes.