I use it for light gaming and school and watching YouTube videos. It works amazing and I am saving up some money to get a GPU and more RAM. Not any problems so far. Fast loading times with the M.2 PCIe SSD.
Interesting part choices...
Congrats on the foward thinking mentality and getting a z370 instead of a locked chipset, now you should be able to upgrade to a 9900k in the future for a large performance gain over the 8100.
Someday i will be able to get a 9900K. But it is over kill for my needs. All i need is ram and a graphics card. and possibly a CPU like a 8350K or 8600K. i don't know yet.
i wouldn't worry about the cpu so much, the i3-8100 is plenty capable. the main things i would focus on is the gpu then ram depending on what you want to play. If you are just looking to play csgo or league of legends get the 2nd ram stick and bump it to 8gb first. It will also make using it as a "computer" much smoother like web browsing and youtube videos (especially with the dual channel memory). If you don't mind me making a reccomendation on the gpu the rx 470 on ebay regularily goes for less then 80. with that and 4gb more ram you would be set for playing anything on the market.
I am mostly playing Minecraft and other small intensive games like GTA V but Minecraft i get 60 fps normally but i need some more beef in my system to push the frame rates especially using mods that are more demanding. But the first thing that i really need is another stick of ram and then a graphics card.