Your problem is in understanding me " So a graphics card can be much more capable than a CPU, especially in gaming terms ".
In 95% of the cases and this topic is totally relevant as this is supposedly a gaming build , The GPU can't bottleneck a CPU . A bottleneck is defined as something being held back in its performance due to lack of power in other area .
let's take an example here : If you pair an i7-4790K with an r7 240 , does that mean that the r7 240 will stop the 4790K from fulfilling its potential due to bottleneck from the GPU ? According to you its a yes.
In real world terms , its a no as the CPU will still be able to do its tasks quite normally and won't be slower in cpu dependent tasks at all further more the problem here is just a miss-pick rather than a bottleneck.
If you reverse the situation and pair a GTX 970 with an athlon as you've mentioned before then in the GPU dependent tasks like games the CPU will hold the card back as its not able to match the speed of the GPU .
The other 5% of the cases I left earlier are mostly cases regarding encoding , decoding and rendering and in that case you're far better off using the CPU alone and you're given the option to do so , which will more or less give you the same performance if you used the CPU and GPU together .
In games you're not given the option , you can decrease bottlenecking by increasing the details and raising the textures quality along with the resolution in order to shift the focus from the CPU to the GPU.