ISA cards, like the soundblaster had the MIDI and joystick on the same connector. The joystick had up to 4 analog inputs, maybe that's what's meant here.
Er, what? Not every IBM PC (clone) had a (sound) card with joystick ports, but almost every PC clone had (ISA, later PCI) extension slots. There were/are plenty of digital i/o adapter cards for those available, from plain, cheap 8255 based ones to those with slave CPUs (potentially more powerful than the host's CPU).
But then, perhaps I don't understand what you mean by 'intentional' GPIO.