I think this is stated confusingly.

gVisor and Firecracker are nothing alike. gVisor is an application kernel written in Go. Application kernels will make your system much slower and can only solve a subset of security issues that would affect the kernel. They're like a kernel proxy of sorts.

Firecracker is a KVM virtual machine but made much more efficient. It's the most desirable of the two. Firecracker and KVM don't require bare metal, but in a virtualized environment they will require nested virtualization support. Nested virtualization is supported by just about every hyperscaler.

The reason KVM is preferable is because it provides strong isolation of the host kernel interface and independent emulation of host devices.

I've built systems out of both and they are quite similar, using different mechanisms as vectors towards the same outcome, which is to minimize shared kernel attack surface in untrusted workloads.

Firecracker doesn't require bare metal, but (1) you want it for performance and efficiency at scale and (2) you take a security hit for nesting.