When to use Rician fading vs Gain for Directional Antenna
The gain is given by the antenna used, and for directional antennas the gain would be much greater than 1. $G_T$ is typically given as the antenna gain compared to an isotropic antenna which radiates or receives equally in all directions.  (So $G_T=1$ corresponds to a perfect isotropic antenna, not a directional one). 

For example, the azimuth pattern below is for a Yagi antenna with a gain of 15 dBi, which shows the highest gain is achieved when the antenna is pointed in the 90° direction, along with other sidelobes that are also sensitive but significantly less gain (for example approximately -4 dBi at 45° and 135°). Not shown but the elevation pattern also has sidelobes that approach 0 dBi. 



image source: Cisco white-paper on antenna patters 

Rayleigh fading is a statistical model for the channel with multiple reflected propagation paths without any particular path being overly dominant, while Ricean results is the model for the channel that has multiple reflected propagation paths along with a dominant (usually direct) path. Rayleigh fading is typically a reasonable model for heavily urban areas while Rician is often used for SatCom. That said, a highly directional antenna can have the benefit of minimizing reflected paths from other spatial directions and therefore can make an otherwise Rayleigh channel appear to be more Rician, assuming that the antenna is pointed at a propagation path that is significantly strong enough, and the environment is not one that would still result in multiple paths of random delay within the main lobe of the antenna without a single dominant propagation path. 

So a highly directional antenna can make a Rayleigh channel into a Rician channel, but that is not guaranteed to do so. It may seem strange at first to state that the antenna can change the channel, but this is really no different than all the buildings, mountains and other objects causing the reflections that also create the channel: the presence or absence of reflections is what creates the channel and the type of antenna is just one of the objects considered in determining the reflection environment.

For further discussion on Rayleigh vs Rician fading see Rayleigh fading with frequency selective fading channel