Almost any CAM editor like CAM350, Gerbtool, Camtastic, etc, can import a DXF file, and export a Gerber file.
Likewise, almost any EDA software such as Altium, Mentor, Cadence, etc, can import DXF files and generate Gerber files.
There's also a free program available from Sonnet that can do the job. You have to register to get it at : **broken link removed**
If you have trouble finding a way to do the conversion, PM your file to me and I'll convert it to a Gerber for you.