That's entirely dependent on what the work being done is. It's typical for gevent users to use the built-in queue systems to pass messages between coroutines, similar to Erlang's actor model. I do t believe a lot of work would be required if gevent users stuck to this style, as gevents docs suggest.
The guarantees of the framework no longer exist when you want to do things in parallel. Sure, not every line of code using gevent would break from this, but that's not the point. Even people using queues are often still playing with shared resources, which would be unsafe in multicore still.
