The basic issue here is that HFSS and the associated files are 3D in nature. A Gerber is used to fabricate a printed circuit and is basically 2D. HFSS supports a variety of file formats. The one useful here is a *.dxf which is supported by HFSS . This file is convertible to a Gerber using software like PADS. Keep an eye on the format as the way HFSS treats the export varies a but from one version to the next.
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Regards,
Azulykit