I am honestly not sure if we can grow gentian in New England or even if it grows wild. I think there are some false friends growing in the woods out here. I used to bump into it growing wild when I lived in Germany in the alps and I started to fall in love with it as a medicinal herb when I lived out there.
I personally buy gentian root in bulk from Mountain Rose Herbs -
https://www.mountainroseherbs.com/products/gentian-root/profileI have also heard that angostura bitters (that we use in cocktails) has gentian root in it, but the company is somewhat secretive about the actual formula.
I "grow" a lot of dandelions too. Or rather, they're in the lawn and since our whole yard is organic, we just leave them be. I dig out some of the roots in October, scrub them clean, and dry them.