MeshCAM

MeshCAM is a professional app that allows to generate gcode. This app works with almost every 3D CAD program by opening the two most common 3D file formats, STL and DXF.

If you just want to make 2D parts without using a 3D CAD program, you can load 2D DXF files (in R12 format) and extrude them to a 3D part.