Assuming you mean concurrent rather than parallel as the GIL prevents you from running Python code in parallel. How does the GIL make things easier compared to the finer grained locking found in Jython which allows parallel execution of Python code in threads?

Threads may still be switched between instructions every sys.setswitchinterval milliseconds so every instruction boundary is still a potential thread switch point, leading to all the issues described by Glyph in his Unyielding essay: https://glyph.twistedmatrix.com/2014/02/unyielding.html