A gear casing heat exchanger has a mounting flange separating the heat exchanger into a first side and a second side. A manifold section is on the first side, and includes an inlet manifold and an outlet manifold. A heat exchange section is on the second side, and extends from the mounting flange in a lengthwise direction. A sealing perimeter is on the second side, and the heat exchange section is entirely contained within a projection of the sealing perimeter in the lengthwise direction. The heat exchange section of the heat exchanger can be inserted through an aperture of a gear casing to provide a cooled gear assembly.