Typically, windmilling of the fan poses no problem for the gear drive.  During windmilling, the speeds are typically low and the load on the bearings is light, so the existing lube film is adequate.
Please note that a windmilling fan in the reverse direction should come to a stop prior to energizing the motor in the forward direction if the motor control is not either “soft start” or variable frequency drive (VFD) to prevent peak shock loading from being transmitted into the gearbox which could cause permanent damage.