I think GT's has something to do with the kiwi juice they use when fermenting. I believe they use that in place of sugar. Their black tea to green tea ratio will also impact the taste.
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I noticed a huge difference in flavor between the commercial tea I get here in the US compared to the tea I got in China (China's tea was better kombucha flavor), so tea source is a factor I think. I have more tea coming from China in 2 weeks, so I'm holding off my brewing until then. I still have my last batch bottled and sitting in the fridge, untouched because it's disgusting. I made the mistake of flavoring with my mom's beet jelly... so not a good idea... I can't bring myself to dump it out though.