From an isolation perspective it's not more secure than a VM, but less, because GVisor needs to implement it's own security sandbox to isolate memory, networking, syscalls, etc, and still has to rely on the kernel for various things.
It's probably more secure than containers though, because the kernel abstraction layer is separate from the actual host kernel and runs in userspace - if you trust the implementation... using a memory-safe language helps there. (Go)
The increased introspectioncapabiltiy would make it easier to detect abuse and to limit available resources on a more fine-grained level though.
Note also that GVisor has quite a lot of overhead for syscalls, because they need to be piped through various abstraction layers.