Hello
If it is a simple Gaussian, the you can probably just calculate the spot after propagating this distance if you know the beam waist and focal length. Look at the beam options tab in the following article.
https://support.lumerical.com/hc/en-us/articles/360034382854-Plane-wave-and-beam-source-Simulation-object
To propagate more complicated fields through free space it is best to use Lumericals far field projection methods. If you want to look at the result of passing through a lens system the I would use a ray tracer like Zemax. FDTD is very expensive, and without anything to interact with the fields will behave predictably, so FDTD would be overkill.
To do this please refer to the farfield documentation. You can specify the plane of interest, then call farfieldexact on the Gaussian beam profile, then import this a a source for second FDTD simulation.
Best
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