bottleneck is not a good word...
For different game, it has different requirements on CPU and GPU, some are more CPU intensive than GPU, there is almost no game will get 100% CPU and 100% GPU usage at the same time.
From power of the two, 1050 ti is way less powerful than the best 1080ti, compared to 8400 is not far away from 8700k.
It all depends on what games you play, 1050 ti is a budget 1080p gaming gpu, can allow med maybe high setting 60 FPS in AAA games. 8400 can handle ~ 100 FPS. In that sense, GPU is weaker. However, if you play games like CSGO, LOL etc, those games are not that gpu intensive and can reach ~200 FPS or more. What games do you play? and what monitor?