We dont have any special output for Robot Millenium, but it seems that this
program is owned by AutoDesk, then probably is able to import DXF
geometries.
Try to export the mesh as DXF-SAP2000 like format, with Files-Export-Using
template-DXFSAP2000
Check in Robot the list of available formats that is able to import, and see
if GiD is able to export some of them.