Instead of using bulb, I used a heat blower to re-flow the chip.
From my observation, there is a the gap between the GPU and the heat sink was join by a burned thermal conductive silicone sponge.
I replace this with an aluminum foil folded a couple of times to get just the right thickness.
After reassemble it back together and now, the moment of truth, Hey! it works! :-)
Thanks a lot jasonshay2. You really save my day.