the blowtorch is designed to burn things, melt things, heat things untill they fall apart, depends on the need, some even use a blowtorch to solder copper tubes fro water or natural gas inside houses, needing 350°c or more, some go up to 500°c.
the heatgun is designed to deliver hot air over a small specific area in a very slow controlled way, so solder points just melt and resolder if that was the problem, the gun must be used caregfully, and set to a specific temperature, 280°c. some use 300°c, depends on the skill and the use, but must be done with flux, in a controlled manner because you can desolder accidentally other parts, and is a task only menta for skilled people with the right tools
when you used the blowtorch is very likely you used more heat than 400 degrees celsius, efectively melting, burning or cracking the gpu die and the pcb itself together with other components, so basically you overcoocked the poor gpu, it is dead, take it to a recycle center and start saving for a new gpu, that hopefully you don't decide to destroy with a blowtorch, something you should never have done, because one has to use common sense, if no one does it there must be a reason, your personal belongings should not be under a blowtorch, never
ask before proceed with a blowtorch is a excellent idea, we would have said to you DO NOT DO IT
if the gpu was covered by a warranty, be 100% sure it is not covered anymore