Fortunately GDSII to DWG (or DXF) are fairly straight forward. If possible please let me know the following:
1 After importing GDS-II, were the geometries displayed in ACE? If no, please make sure you have selected the "Top Cell" before export. Please view the following link, https://numericalinnovations.freshdesk.com/support/discussions/topics/14000003110
2 What is the size of the resulting DWG file? Is the DWG file size 0 bytes, or is there something in the file? If the DWG has a decent file size, maybe your GDSII patterns are in another block within the DWG (typically this occurs when the correct top cell is not selected). In AutoCAD or another CAD tool you can select another block.
3 Have you tried exporting to DXF? DXF is a simple text format, and more common for other CAD tools to read.
4 If possible, try converting the sample GDSII file shipped with ACE called "Abacus.gds" (it's located in the ACE folder .../examples/gds). What that successful?
Feel free to send us any sample GDSII file and I'll be happy to created a tutorial movie showing exactly how to perform the conversion.
Lastly here's a somewhat related topic on the support forum: https://numericalinnovations.freshdesk.com/support/discussions/topics/14000002751
Thanks again, and I look forward to hearing from you.