All GPIO (not GPIOTE, that's something slightly different!) functions (HAL) are in nRF5 SDK path \components\drivers_nrf\hal\nrf_gpio.h. Simply configure PIN as input (e.g. by nrf_gpio_cfg_input) and then read it by nrf_gpio_pin_read any time you want. If you want to get interrupt when PIN value changes then you need to go with GPIOTE. Both these features and HAL functions are explained in nRF5 SDK examples, just find it and read it.
