x11-libs/wxGTK-3.0.3 is currently the latest version in the core-kit/1.2-prime branch. wxGTK is relied upon by dev-python/wxpython, but wxpython is very particular about just which versions of wxGTK it will work with - they must match versions pretty much perfectly. Presently, they work together with 3.0.2.0; I have wxGTK using 3.0.2.0-r3 and wxpython using 3.0.2.0, which is also its latest available version in portage.

wxpython does not have this version check as part of its build requirements as far as I can tell, but rather checks at runtime. This means that affected packages, such as playonlinux, will appear to emerge perfectly fine but will then refuse to run, with a message complaining about the mismatch. This is not unique to playonlinux, of course, and I can personally verify that pcem suffers similarly (that program is not in the repos at present, but easily enough acquired and simple to build).

As such, it would seem to be best to have a small kit for these in order to ensure they stay properly matched, as per TemptorSent's suggestion on IRC.