Looking at this
Arduino driver, I think you need to select a range for the gyro and possibly wake it from sleep. According to the
register map, that means writing 0 (250 degrees/sec), 1 (500 degrees/sec), 2 (1000 degrees/sec) or 3 (2000 degrees/sec) to bits 3-4 of the GYRO_CONFIG register (0x1B). To wake it from sleep, write a 0 to bit 6 of the PWR_MGMT_1 register (0x6B). You also might want to change the clock source to one of the gyros to increase accuracy.