Yes it could be the transmission ECU and it is located next to the engine ECU in the enclosure in passenger side rear of the engine compartment. When the gearbox fault warning comes on the TCM (Transmission Control Module) has detected an electrical or mechanical fault. Since that warning is on there will be fault codes stored in the TCM that need to be read with a compatible scan tool. The usual failure of the transmission is a burnt or cracked forward drum/clutch. Since the gearbox fault light is on the code logged is most likely a P1722 for stall speed. This is the weak area of this ZF transmission. The electronics are solid. Of course I can only give you what I see and we will need the fault codes to properly diagnose the problem. Let me know if you have any further question or concerns. We should check the fluid level and condition but it is not usually that simple of a repair. Since there is no dipstick the fluid cannot be checked easily. You will need to get your vehicle on a level surface or four post lift. Start the engine, go through each gear and warm the fluid to about 40 degrees C. There is a level/fill plug on the passenger side rear of the trans case. Remove the 8mm hex/allen plug and fluid should flow out with the engine running and fluid warmed to temp. If it does not flow out then the fluid needs to be filled through that hole