dBi as an antenna measurement mean's dB over isotropic. An Isotropic radiator is an antenna that radiates equally in all directions. In other words, the radiation pattern look's like a perfect sphere.
Because antenna's are directional, the antenna directivity is measured as the peak of the radiation pattern.
A dipole antenna is, I beleive, +3 dBi in the peak direction. On the other hand, the radiation pattern look's like a donut, with no radiation propogating from the top or bottom.
FOr a +17 dBi Antenna, you will probably need some sort of dish, and you will end up with a very directive antenna, something that look's much like a pencil beam.
In other words, for +17 dBi gain, you won't get a 120 degree beam width. Just a quick guess, I'd expect to get at most 4 to 5 dBi of gain. (Think geometrically about the power direction)
Dave