Hi McGwire,

The Source Ellipse only sets a Gaussian distribution for the angular spread of the rays. This source has a spatial size, i.e., the launching surface. The position of the rays launched from this surface follows a uniform distribution. If you place a detector immediately after the source, the angular spread will be Gaussian but the positions will be like flat top. 

For modelling a Gaussian beam, if you need a point source that emits rotationally symmetric Gaussian distribution of rays, then Source Gaussian offers a simple model.

Another option is Source Diode. It gives you more freedom on how to define the Gaussian distribution. With Source Diode, you can assign Gaussian distribution to both the spatial and angular distribution of the rays. You can also have different emission areas on x and y, and different x and y far field divergence.