What's the best workflow for creating gerber files from, say, an HFSS project?
The method I have sucessfully used is as follows:

1) Export DXF from HFSS

2) import DXF into Turbocad - smash down to ensure that the DXF consists only of the polyline primitive type.  Also I use the join polyline function in Turbocad to ensure the polygon is closed.  You may have problems with enclosed holes (like anulus) which may require to be split as two polygons.

3) Import processed DXF into GC Prevue (not the free version, but the puchased one that allows conversion) - available quite cheaply, only $100 or so.

4) Export from GC Prevue as gerber.

I have also imported directly a DXF consisting purely of primitives into Pulsonix (my preferred PCB tool) and PADS - although you may have to specify the DXF as being a filled shape.

Not being an Eagle user, I can't help you with this particular tool, I'm afraid.