They both have panel filter covering but the GT's is different (thicker?) to that of the ST's resulting in a slightly darker screen as insane200 says. Plenty of people have noticed it. The plus side is that the GT can handle brighter lit rooms better than a ST as a result. Although in my lounge I have had no washed out blacks in daytime viewing. Reflections/glare is more of a problem for me.
I had my ST calibrated by a friend in the trade last week and even he was surprised how good the picture is for the price. It's a TV that needs some proper tinkering with to get the best out of if you can't get it done properly but it's well worth it and any difference in picture quality with a GT is negligible. (2D) The biggest downside I have found with the ST is it's limited amount of preconfigurable settings you can have. It's a shame I can't have 3 (day, night, sport) well set up. With the ST you are limited to 1 and a bit (OK you can just about set 2 up but the 2nd will always be sub standard.
Great TV for the price though and i'll be interested to see how mine ages.