Based on your description, it sounds like there may be an issue with the blend door or actuator that controls the flow of hot and cold air in your Golf GTD. This can sometimes cause one side of the vehicle to blow warm air while the other side blows cold air.
Before flushing the coolant system or changing the thermostat, it's best to have the blend door and actuator checked by a professional mechanic or VW dealership. They will be able to diagnose the issue more accurately and recommend the necessary repairs.
If the blend door or actuator is found to be faulty, they will need to be replaced. In some cases, it may be possible to perform a recalibration of the blend door actuator to reset its position and fix the problem.
In the meantime, you can try adjusting the temperature settings to see if that makes any difference. If one side of the vehicle is consistently blowing cold air, it's possible that the air conditioning system is stuck on that side. Try turning off the air conditioning and see if that improves the airflow from the passenger side vents.