Netlab reports that the water comes from Mount Hotaka in Gunma Prefecture, the sake-specific rice variety is Maikaze which was first grown in Gunma Prefecture, the yeast is the G2 strain first developed and cultured in Gunma Prefecture... you get the idea.
The sake is a yamahai junmai ginjō, made with resident lactobacilli from the Tsuchida brewery, founded in you-know-where in 1904. Even the label carries on the theme - the outline of the prefecture is said to resemble a bird with outstretched wings, so the label has a detailed drawing of a flapping crane. The Netlab article also comments that yamahai has the reputation of being an acquired taste and hard to drink, but Gunma is pasteurised once to make it mellower and more palatable.
It will be sold by Kurand, a chain of sake shops in Tokyo offering "all you can drink" plans at their Kurand Sake Market outlets. Kurand's information page also emphasises that it was an attempt to create an easy to drink yamahai. The soft water from Mount Hotaka is ideal for yamahai brewing, and using the lactobacilli in the Tsuchida brewery helped to create a flavour that is uniquely Gunma. Tsuchida's tōji, Genki Hoshino, comments that every brewery has its own approach, which yields an infinite number of expressions of sake, making it a fascinating drink. He wants to rid himself of preconceptions and create something that others will thoroughly enjoy.
Kurand describes the distinguishing characteristic of Gunma's flavour as gentle umami with depth, instant acidity, pleasantly short finish with a gentle, sweet aftertaste reminiscent of pineapple or melon. They advise trying it chilled straight out the fridge, then tasting it again a few times as it comes up to room temperature to see how the flavour changes, or heating to 50-60°C. It's also recommended to drink with meals, pairing with foods with a bit of oil like nasu agebitashi (aubergine/eggplant soaked in dashi-based broth and deep fried) or fried chicken (tori no kara-age), or else with acidic foods like cheese to bring out the rice-based umami.