A gear unit comprises shafts (101, 102) for connecting to an external mechanical system, at least one gear stage (103) between the shafts, a lubricant oil room (104) for storing lubricant oil, an oil channel (105) for transporting the lubricant oil from the lubricant oil room to the at least one gear stage, and an immersion heater (106) for heating the lubricant oil flowing in the lubricant oil room towards an inlet of the oil channel. The gear unit further comprises a mechanical flow guide (108) located in the lubricant oil room and configured to modify flow of the lubricant oil flowing towards the inlet of the oil channel so as to increase flow velocity of the lubricant oil on the surface of the immersion heater. The increased flow velocity on the surface of the immersion heater makes it possible to use higher heating power without damaging the lubricant oil.