What you are saying has nothing to do with what is in the video. This is not fluctuation in brightness issue. It’s near-black gamma shift that exists regardless of framerate or frame spikes. You can even see the difference between gsync on/off in a static picture.
The brightness at static 120 hz will be different than whatever framerate your refresh rate is matched to.
Just adjust brightness with gsync on with a framerate you can hold and the black levels will be fine.