Hi,
Indeed, STL file format could be appropriate. You can also take a look to STEP format (I am not familiar with Autodesk but I guess exporting to STEP format might be an option since it is pretty standard). If you have labels in your STEP file for your subdomains/boundary, you may be able to import it into Gmsh (see this post).
Regarding support, Gmsh has a mailing list
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