It was just ellipsoids. I don’t know if any game specifically used Gaussians. But, the idea of splatting points, Gaussians, ellipsoids and a variety of other shapes has been around for at least 20 years.

The novelty of the paper was in using the differentiability of Gaussians to enable fitting splats to incrementally match all of the target photos simultaneously. So, it’s a new way to generate the splats from photos rather than modeling them by hand.

The backgrounds are static and prerendered! There's one sphere .bmp that's scaled and stretched. It comes with a depth offset map that populates a simple z-buffer to prevent overdraw. So rendering each frame becomes just a couple hundred dozen operations!