I have Asus GTX 780 Ti Directcu II OC GPU. It's overheating while playing games. Temps are 82-85 degrees celsius. I've overclocked GPU. I've never putted thermal paste on my GPU since i bought it. I'm afraid to cause damage on GPU , fans or pcb.
You're right to be concerned. Your GPU processor does not have a lid (integrated heat spreader) like modern CPUs. It's just silicon staring up at you. You put uneven pressure on it and crack ... dead (I killed an Athlon XP doing that). That said, I followed the linked guide and brought down my temps by 10 degrees. The rest of the stuff is a lot harder to damage.