I can't speak for the GT50. However, I have had the 55ST50 since early April and love the thing. I am a young professional who works about 7-6, but will from time to time have some 2 or 3 hour video game sessions at night after work and have had a couple 5 hour sessions on the weekend to veg out and relax. My games include NHL12, MLB2k12, and CODMW3 on 360
At this point, i have never had any burn in issues whatsoever. I know that the 100 hour panel prep has next to nothing to do with burn-in, but i didn't do it. So before you ask, if you were going to, no need for it unless you want to replicate certain calibration settings. I would also note that i have never noticed even the slightest input lag. I have taken tvs back in the past for this and usually pick tvs with little input lag and will compromise features to get tvs with low lag. Luckily with my ST50, i didn't need to compromise anything and still have no lag.
One thing worth mentioning, i don't pay a lot of attention to the GT and VT threads, but my understanding from early reviews is the GT was a marginal upgrade over the ST, but not worth the money. However, the VT was worth the money over the ST. This view may have changes since i was researching in early April, but it is worth considering. Depending on you budget, you may be better off saving some money and getting the ST or paying slightly more and getting the best Panny has to offer.