Antenna gain is calculated based on a known standard called an Isotropic antenna. This type of antenna does not exist but would be one that equally emits energy in all directiosn with no loss points. Only the sun can do this. With any normal antenna you have a cable connection point which is a null and the opposite side which is also a reduced signal level location. Depending on the gain of the antenna, if it is an omni, the higher the gain the flatter the donut shaped coverage around the antenna. On a directional, the more narrow the beam that the antenna creates but vertically and horizontally.
The advantage of using an antenna is that the link-budget is effected in a positive way in both directions.
BeamFlex is an intelligent antenna array of directional antennas that transmit omni directionally when beaconing but when transmitting to a specific user will send all the energy in one direction, towards that user, and each user when it is their turn to recieve data from the ap.
You may want to review the BeamFlex training video on the training site, https://training.ruckuswireless.com/course/view.php?id=812
. You can also find several on youtube.com, https://www.youtube.com/watch?v=BvIu_g3nN4g&t=17s
If you use an external antenna on an ap you loose the intelligent antenna array and only get the capabilities of the external antenna you are choosing.
Hope this helps.