Rapidly poll GPIO Input

I'm trying to use the GPIO to receive a fiberoptic input. Is it possible to rapidly check the GPIO input using a clock and or interrupt?

I've been doing some reading, and it looks like I'd use SYSCLK4, but I'm confused at the limitations and implementation of creating an interrupt to check the input at the GPIO pin.

Is it possible to sample the GPIO input at 1 MHz? If so, would I use a GPIO interrupt, or would I initialize a clock signal using SYSCLK4 and the timer.h/timer.c files?