Its actually very simple to get a screenshot of ur JOGL renderings. Here's a small snippet that could be used inside (at the end) of the display() method of ur GLEventListener:
BufferedImage tScreenshot = Screenshot.readToBufferedImage(0,0, tScreenWidth, tScreenHeight, false);
File tScreenCaptureImageFile = new File(tFileName);
ImageIO.write(tScreenshot, "png", tScreenCaptureImageFile);
... or u can press the whole process in a single line