Someone has to produce content for new tech.
Ppl probably have 1080p 120hz. Maybe possible.
Either way, the assets for GTS are meant to be future proof and Polyphony do now outsource content creation.
So GT7 could arrive sooner rather than later.
HDMI 2.0 does support 1080p at 120hz, some 4K tvs (very few) do accept 1080p 120hz input. Not 4K at 120 hz though, for that you need HDMI 2.1 which is brand new
So you need one of the few hdmi 2.0 4K tvs that support 120 fps at 1080p, or a brand new one with hdmi 2.1 that supports 120 fps input. It's a niche inside a niche.
If you have a PC you could test your tv
However this was never about dropping back to 1080p to get 120fps, 4K seems to be the primary goal.