I think you mean a GT50; I don't think there is a GT60 (at least not in the US). Anyway, have not seen anything like that reported here. The reports of image retention are usually from station logo's or the side bars from 4:3 content. When people get IR they are usually advised to do two things. One, stop doing whatever it was that caused the IR. Two, watch full screen content until it fades away. There are also a handful of other things to try but I've not seen any indication they work any better than viewing full screen content.
If you call Panasonic DON'T make any mention of burn-in or image-retention. Refer to the issue as a "screen irregularity" or "non uniformity". There isn't much you can do if a tech shows up and classifies the issue as burn-in, but you certainly don't want to start out by telling them that it is.