Both Jython (Java impl) and IronPython (C# impl) works fine without the GIL. The big issue is compatibility with C-based libraries, as I hear Larry Hastings, there are three levels of compatibility:

1) Fully compatible, nothing needs to be done 2) Fully compatible, but a recompile of C-libs is needed 3) Almost compatible, but some updates to C-libs are needed

Larry is aiming for 2)