Doesn't make a bit of difference whether you use blue or amber. I use the amber just because thats what everyone seems to use but the blue are fine.
I don't know how to tincture gymnema, I don't know if ACV or Vodka or a combination of both is used or not. The only tincture I know of that uses both is the Lobelia, I'm sure there are others though.
Have you tried the middle isles at Walmart or K-Mart? They put them in the middle isle and put them on sale at the end of the season where I live.
Yep, tinctures can tincture forever! Once a tincture is done I get enough out to fill a couple or three 4oz. bottles and leave the rest tincturing. That way it just gets stronger.