Is it true that in multiprocessing, each process gets it's own GIL in CPython? How different is that from creating new runtimes?

The short answer to the first title question is: Yes. Each process has its own Global Interpret Lock. After that, it gets complicated and not really as much a Python matter as it is a question for your underlying OS.

On Linux, it should be cheaper to spawn new processes through multiprocessing rather than starting a new Python interpreter (from scratch):

• you fork() the parent process (side note: clone() is actually used these days), the child already has your code and starts with a copy of parents address space -> since you are actually spawning another instance of your running process no need to execve() (and all the overhead associated with that) and repopulate its content.
• actually when we say copy of the address space, it actually does not get all copied, but would rather use copy-on-write; so unless you modified it, you do not need to copy it at all.

For that reason my gut feeling would be, multiprocessing would almost always be more efficient than starting a completely new Python interpreter from scratch. After all, even you started a new interpreter (presumably from the running process), it first performs the fork()/clone() including "copy" of the parent's address space before moving onto execve().

But really this may vary and depends on how your underlying OS handles creation of new processes as well as its memory management.