Hi,
i'm looking for a game demo here which need fast gpu and fast cpu. Can you help me

dunno if it is available for free on steam, but i played this demo Gothic Playable Teaser (seems for owners of gothic series)
bet it would suffice your demands for fast cpu and fast gpu :)

Thanks Franky !

But it looks it is not anymore available to play.

Probably Destroy All Humans! Demo, but you need to hurry up because it's only available until June 15th.

You have Final Fantasy XV, Shadow of Tomb Raider or Wolfenstein on Steam.

If you really just want to test your system, just run a graphics benchmark, like say Unigine Superposition at the same time you run a CPU bench, like CPU-Z or Cinebench.
For a quick test I generally use CPU-Z set to run on not every cores and GPU-Z, wich have a small render test. Infortunatelly, that demo isn't enough. The requirements are very low for modern desktop systems (except the 4Gb VRAM).

Best Game I know on GOG to test hardware is Ashes of the Singularity but don't have a demo. It can tax both the CPU and GPU and tests are very repeatable, there's a reason some people call it "Ashes of the Benchmark".