By default, GD uses an indexed color table.

One simple mapping is to think of the "color wheel" of red-yellow-green-cyan-blue-magenta-red and all the interpolated colors. Map an arc of, say, red through blue and insert those in order into the GD color table.

With only eight-bit color channels, you may need to dither the intensity or the third color a bit if you want all thousand to be unique.