Db gain is not linear.
A higher gain antenna has a longer range, however remember that the antenna in the laptop also has to transmit.
I usually find that a better alternative is to build a free or almost free parabolic dish to control the throw of the signal, and also improve reception of the laptop's signal.
http://www.freeantennas.com/projects/template/
I built a custom one for a friend, a metal framed house was killing his range to about 30 feet. After the build he got full strength on his patio through alot of walls. Another one I built was placed in a utility room at one end of a house 120' long and two stories. It works well.
http://www.freeantennas.com/projects/template2/index.html