To enable GPIO pin5:

gpio_init();
	PIN_FUNC_SELECT(PERIPHS_IO_MUX_GPIO0_U, FUNC_GPIO5); //enable pin5 GPIO
	PIN_PULLUP_EN(PERIPHS_IO_MUX_GPIO0_U);  //Enable pullups
	

Then to enable a timer to callback and read the value:

//===================================
	// TIMER STUFF
	//===================================
	
	//Setup timer callback
	os_timer_setfn(&buttonTimer, (os_timer_func_t *)buttonTimerCB, NULL);

    //Arm the timer
    //&buttonTimer is the pointer
    //35 is the fire time in ms
    //0 for once and 1 for repeating
    os_timer_arm(&buttonTimer, 35, 1);

Then the timercallback function:

uint16_t pinvalue=0;
void buttonTimerCB(void *arg){
	//INFO("Timer says hi!");
	pinvalue = GPIO_INPUT_GET(GPIO_ID_PIN(5));
	if(pinvalue){
		pressCount = pressCount+1;
		INFO("Button pressed #%d \r\n", pressCount);
	}
}