How hot must I set my heat-gun during re-flow?

You should warm up your notherboard ( a Griddle will work fine) until the top of the board reaches about 120C, then bring the heatgun over the top of the GPU and keep it around 20-25cm initially, going in small circles over the GPU - keep it moving. If you can, use an IR thermometer (Thermocouple thermometer would be perfect) to keep an eye on the temps. You want to take the temperature to 230C. When you reach this temperature, hold it there for no more than 10 seconds, at this point the lead free solder balls will be properly melted. Make sure you do not touch anything or move anything against the board at this time until the board has cooled down. Hope this helps, good luck