var savefile = new File("~/Desktop/sample.gif");
var gifOpt = new GIFSaveOptions(); // GIF形式オプション
gifOpt.colors= 8; // 8色
gifOpt.dither=Dither.DIFFUSION; // 誤差拡散
gifOpt.ditherAmount = 25;
activeDocument.saveAs(savefile, gifOpt, true, Extension.LOWERCASE);