GA is an improvement, not a tech. You obtain the trade good by building the improvement. There can be only one GA in the universe. Once built, the thing adds +1 speed to all your ships. (The bonus won't show up immediately, but on the next turn (or two).)
Trading the trade good to another race gives the same speed bonus to that race.
Personally, if I built it I'd never trade it. Speed is life. There is no imperative to build a GA unless you want it exclusively. If another race builds it, a trade for it is usually pretty cheap, so you can at least keep up in speed.