using Screenshot:
BufferedImage image = Screenshot.readToBufferedImage(width, height);
using AWTGLReadBufferUtil:
GLAutoDrawable glad = (GLAutoDrawable) event.getSource();
AWTGLReadBufferUtil glReadBufferUtil = new AWTGLReadBufferUtil(glad.getGLProfile(), false);
BufferedImage image = glReadBufferUtil.readPixelsToBufferedImage(glad.getGL(), true);