Hey Douglas,
you need no special program to write G-code. As G-code consists of plain text, any text editor program will do. After having written your G-code as plain text, save the file with a ‘.ngc’ file ending which will allow it to be recognised as g-code file.
There are however a lot of G-code editors available. Also your favorite IDE may or may not support syntax highlighting for G-code, as an add-on or plugin. If you want syntax highlighting for G-code under Emacs, put this into your ~/.emacs file.
If you want to see a simulation of the G-code you wrote as a 3D toolpath, you can use CAMotics. It is Free and Open Source Software. By the way, after writing CAMotics, its author and his uncle created the Buildbotics CNC Controller.
If you want to learn how to write G-code, a standard work is:
- Smid, Peter: CNC Programming Handbook, Industrial Press, South Norwalk, CT, USA, Third Edition 2008, ISBN 9780831133474
This book will also answer your question in the other thread on what one would expect from a CNC Controller that the Onefinity Controller can’t do. In fact there’s a lot. One example that is frequently requested is the ability to adjust the feedrate during operation.