The game is overall more CPU intensive, but to get the heighest fps you need both compenents to compliment each other. Just because a game is very CPU intensive doesn't mean, that you can get high fps with a weak GPU.
Both GPU and CPU are used to render a frame and the ratio might be something like 1:4 for CS:GO (that ratio is made up by me to better explain my point), but that means, that there is still a limit to how much fps you can get.
If you really want to find out, which component is causing the lack of your fps, then you should monitor your hardware usage with MSI Afterburner and the overlay functionality. This way you can display live information for each CPU core, while the game is running and I recommend to do that while watching a demo and focus only on the numbers, so you don't get distracted by the actual gameplay.
I don't want to get your hopes high here. If people are talking about 250-300 fps, they talk about average fps. You still get drops in very demanding situations, which cause a difference in input lag or stutters.
Download a demo from the E-League Atlanta Major and observe your hardware usage live.
I run a system with an I7 7700K and a GTX 1070 and I see a lot of drops into the 200-220 fps on modern maps like infereno, nuke, train, even when my average are 250-300.
Never trust average fps. For fluent gameplay it's the min fps that matter and cause you to miss shots, because the game is not reacting as fluent as you are used to in practice. That's why even with that monster system, I play with fps_max 144 on my 144 Hz monitor, because I know that my system can maintain that regardless of the situation. That's perfectly fine.
Keep in mind, that CS:GO is build with a very old engine and not very good to utilize 100% of your CPU. So you might notice something like your GPU not beeing at 90-100% and your overall CPU usage not beeing at 90-100%, but you still don't get more fps even if you play with a very high fps_max. Then it's not a limit of your CPU core power, but with the way the game is utilizing your cores and only a more powerfull clock speed can make up for that, because a single very demanding task can be processed much faster with a higher clock speed.
I know that everyone likes the low input lag that you get from having 300 fps and I like that as well, but consistency is more important.
I can only recommend the tech channel Tech Deals . This person has a lot of knowledge about building PCs and he shows live gameplay footage with the overlay feature, so you can actually see real numbers while he is playing the game instead of some average fps that are generated by some software in a specific test scenario.
Take his recommendation with a grain of salt though, because he is mainly focussing on "deals" performance for money for casual gamers and not on getting the most beast high end system to run a specific game at x fps.