I've done GPU reballing ith a hot air gun on G3 laptops.
I bought a replacement G5 as my regular G5 died from faulty ram sockets.
the new G5 had garbled graphics. slightly usable but garbled.
Yesterday I decided to reflow it.
followed all the fregac steps as above and as I have previously done with laptops (fixed 5 in the past).
waited an hour then added thermal silverpaste.
red LED came up!!!
added more thermal paste, again 4th red LED.
this morning I tried pmu reset and change battery and still red LED.
I took the heatsink off and the little bit of plasticy stuff that goes betweeen the heatsink and the gpu and I flipped it over.
replaced heatsink, and a touch more thermal paste and fired up the machine and it booted to GRAPHICAL PERFECTION!!!!
very stoked with the outcome.