The gearbox oil temperature is pretty much the same like the OAT, so you can use it to read the OAT. In the game, it's not warming even with running engines, in reality I have no idea. So the point is you can use it to determine the outside temperature.
A little correction here: I made a mistake, it is the hydraulic oil temp which always shows the outside temp . The gearbox oil temp actually rises with running engines, sorry again I didn't remember correctly :)