> Ruby also has a GIL.

MRI has a GIL; major alternative implementations (JRuby, Rubinius) do not.

OTOH, addressing the downsides of a GIL are not the only reasonable motivations for an alternative implementation, so there's no reason that a better-than-stock Python (or Ruby) fundamentally must remove the GIL (the current "MRI" used to be an alternative, YARV, to the old MRI, and both had GILs.)

True. Jython and IronPython also do not have a GIL.