Gentian seeds are germinating in two weeks with GA-3. I dissolved a pinch of GA-3 in about 1/4 cup of water. I surface-sowed the seeds, and carefully watered them in with a fine mist. I then applied GA-3 to the surface of the potting mix with an eye dropper. The containers were then covered with clear plastic, and placed 6 inches under cool white fluorescents, at roughly 60F. Don't go too much warmer than that. G. sceptrum, calycosa, puberulenta, and clusii are germinating strongly now. I can now grow them on under the lights until April, at which point I will transplant them and place them in the cool greenhouse.
Sorry, I can't provide an exact concentration of GA-3.
As usual, a large percentage of exchange seed is dead. Zero percent germination doesn't leave much to the imagination. GA-3 will not resurrect dead seed. If you really want the plants, get seeds from several different sources. I recommend Alplains, Gardens North, and Jelitto for viable, correctly named seed. Jelitto is not cheap, but their seeds grow. I still have a lot of seed in the freezer from the Robinetts and Ron Ratko, and it still germinates well.
If you've let your seeds lie around in a shoe box for 15 years, you really shouldn't send it in to the exchanges. It's dead after about 2 - 4 years. On the other hand, if you freeze seed at 0F in an airtight container, it will stay viable indefinitely, 20 years or more in my experience. Never expose seed to temperatures above 100F, like in a closed car on a summer day. Seeds are a lot more perishable than you might think.