Ok here we go!
I have no idea what temp the gun is, it's an old Black and Decker 1200w paint stripper gun with about a one inch nozzle. After some investigating I came up with this method that usually works but is certainly not guaranteed and is not a professional reflow by any means!
Dismantle your laptop down to the motherboard being out in your hands, first dismantle the fan and clean out all the crap before you forget and put it all back together uncleaned!
Clean the heat sink compound off the CPU and GPU, lay a layer of tin foil over the board and carefully cut a hole to let the GPU heatsink pad poke through [about half an inch square], try and ensure you cover the tiny surface mounted resistors on the top face of the GPU as if you melt them off you're totally xxxxed!
Obtain a copper coin or similar and smooth off one face, put some new heatsink compound [Arctic Silver] on the GPU and lay the smoothed coin face on it [use a coin or similar that covers the whole top contact patch of the GPU and protects the bits underneath a bit].
Obtain some electronic solder with a flux core built in [or possibly seperate solder and flux if not] lay a small length say a curled up inch on top of the coin and grab your heat gun.
Heat the solder and thus the coin and thus the GPU slowly and carefully from a few inches, maybe closing in and moving back a bit now and then until the solder begins to melt and pool, at this point move in to say a couple of inches for about ten seconds to ensure it's well heated, pull back a bit for say five and turn the gun off.
Leave to cool totally untouched for 15 minutes at least, reassemble with new heat sink compound and send up a prayer!
You can plug a few bits in AND THE HEATSINK and see if the screen comes up before reassembly [make sure the bits are seperated and can't short out obviously]. If there are signs of life turn off and reassemble a bit further, do not be tempted to look at it very longas all those loose parts laying around will tangle and blow you nice repair to hell in a handcart!
If you had some liquid or paste type flux handy you could have put some under the edges of the GPU before heating, it will help a lot and may be the difference between success and failure.
A long lasting repair is not at all guaranteed although this Samsung seems ok at present whereas my other HP DV2000 model has just packed up again [for the third time] after about six months of use.
The idea of the solder is obviously to give some idea of temperature, don't be too keen when it melts as too much heat for too long can never be undone and will only melt all your solder balls under the GPU into a lump and bang it all goes.
May the force be with you if you decide to have a go, have a look round on Google and You Tube before you start also for dissassembly info, the first laptop strip down is always the hardest one! There's a lot of information out there. If I can help any further shout out.
Regards, Bob Green.