Hey there Dawg,
I'm actually returning a VT50 for another this evening due to issues appearing to be coming from a faulty transformer/ps issue on the unit.
In answer to your question , the GT and VT are using the same panel, the ST has a different one. These are using a higher color gradation scaling (24,000? somewhere around there). Significantly higher than the ST panel. The VT also has some other differences, like a dualcore processor (mainly for internet apps) and a louver system to aid with brighter ambient lighting issues. If you have the money to spend on it the VT is a better unit, but by no means is the GT a slouch.
I've been looking at the VT series for awhile and still even with the PS issue feel it's a great tv. There's quite a few people (much more experienced than I) that would agree.
There are some things to look out for on this unit. One of them is a "buzzing", but this can also be misdiagnosed by loud fans. I am able to hear clearly the buzz from 25' away. Clearly a defect in my particular TV. One way to tell is use a toothpick to stop one of the fans in the back and see if the noise goes away. The second thing appears to be an issue with the panel manufacturing where there is a "band" located on the right side of the screen. It is believed this is coming from rollers on the assembly line contaminating the panel and leaving a residue which results in uneven pixel excitement. Thats something that is never going to be fixed if you get a unit like that (outside of swapping it out). Easy way to check for that is burn a USB stick with burn.
Outside of that... not a bad tv choice...