How can it be optional if there is lots of code that assumes you are using GC? For example, as far as I can tell the stdlib doesn't do its own allocations. Does this mean you can't use the stdlib with GC disabled? Or am I missing something here?

A friend of mine wrote a DSL for audio using Nim with GC off.

https://github.com/vitreo12/omni

I don't know enough to comment but it may be useful to look at things in the wild. This project also heavily relies on calling in C to interface with environments and the SuperCollider scsynth.

overally if you write an os you need to write your malloc/free .. and then you should be able (i think..) to use many of the gc-s anyway and more of stdlib.

But you can also think of Nim as macro-able higher level C and write it like that (but indeed you probably still need this minimal allocation support)