For gtk, gstreamer etc. PyGObject should just work. It has PyPy as part of its CI pipeline. Also in combination with matplotlib (GTK3Cairo backend) and numpy.
Not sure about PySide2..
Not sure about PySide2..
Therefore until and unless PyPy has usable Qt bindings (e.g. PySide or PyQt), then unfortunately I need to stick with CPython :/
Here is the only PyPy related issue I found for PySide2: https://bugreports.qt.io/browse/PYSIDE-535
PyQt doesn't have a bug tracker ¯\_(ツ)_/¯