I have the regular GT King (not the pro), currently running the latest stable release (9.2.5). In all videos that I have checked, the box will do the upscaling to the configured resolution (UHD), and as far as I can tell it does a pretty good job. You can disable the upscaling by whitelisting the resolutions supported by your TV. If the video matches any of those resolutions, then it will be sent in the native resolution. (You do not need to do this for framerate, that is adjusted automatically by default).

I am not so sure if the upscaler is always software, or if it uses hardware in the Amlogic SoC. Either way, I am pretty satisfied with the result.

The only caveat you should have in mind is that the “Noise reduction” option is enabled by default, and that may cause some juddering/ghosting during fast moving scenes in UHD (it’s not just the motion options of the TV). Disable that and the motion issues will be gone.