When I brewed a gruit ale a couple of years ago, it was extremely tart, and tasted like a lambic. However, the tartness was not from Lactobacillus but rather from the choice of herbs used. It seems that one or more of the traditional gruit herbs (yarrow, I think) adds a great amount of tartness, moreso than bitterness and in addition to the obvious floral herbal flavors. I believe this is appropriate to the style, as the tartness offset any sweetness from the malt, to where it tasted far more tart than sweet. A sweet gruit is certainly possible, but I feel it is not necessarily the norm. The norm might be more on the tart side. But I haven't tasted any historically brewed versions to know for certain. I would question New Belgium as I'd guess they are far more interested in selling an easily quaffable beer than brewing with historical accuracy. And... Fraoch is not a true gruit in my book either. Close, but... it's only heather, and not even strong heather. Plenty of room for exploration, though. Gruit is always an exciting thing. Like the "Mystery Flavor" in Dum-Dums suckers or "a box of chocolates... you never know what you're gonna get!"