That’s not GPU passthrough. That just enables VirGL, which is a translation layer that passes some OpenGL calls through to the host’s Mesa installation. It has rather poor performance though, it’s extremely limited and is rather buggy too. You certainly can’t use it for cutting edge gaming.
GPU passthrough is when you pass through an entire GPU device as-is to the virtual machine. That is: if you have an Nvidia RTX 3060, the guest operating system will see an Nvidia RTX 3060 and it can use the native drivers for it. This gives you near-native performance for gaming.
Now, I didn’t even know this was possible with VirtualBox (if so: cool!), but it’s certainly doable with KVM if you have the right motherboard and GPU combination. I have done it, but it is quite the hassle indeed though that isn’t really KVM’s fault.