RLOD is tricky. If the system has never had it before, you can try opening it and cleaning everything. When you have the motherboard cleaned I put it on some wood blocks on top of a griddle and “cook” it at 350F for 20 minutes. After I use a heat gun and blow hot air directly on the cpu/GPU for about 2 minutes, then let it sit over the griddle for another 10. Finally I place the motherboard (still on the blocks) in the oven which was preheated to 350 and turn the oven off. This lets it cool down very slowly. When rebuilding the PS3 I use arctic silver 5 for thermal paste. Put the whole thing back together and hopefully it worked.
If the system has had RLOD before you can try the same thing but the chances of success are lower. If this fails your only other option is to try reballing the GPU... or just buy another PS3 from LetGo and start over.