Can features like ADC and PWM on a pin be considered as functions/configurations of a GPIO?
No, because you can have ADC or PWM capabilities independent of a GPIO. E.g., the 32-pin variants of the ATmega328 have two pins that are exclusively ADC inputs (pins 19 and 22) without any digital I/O capability. So you can configure pins as PWM, ADC, GPIO, etc., but GPIO is a discrete concept from the other functions.