Maybe in the maker and low end hobby community, but not here.Now everyone and his dog prefers the Arduino UNO for driving home routers
You'll find very few posts here from people using Grbl and Arduinos to control machines.
I believe that LinuxCNC may be the only free option, and you don't want to go there.What can I use to - convert g-code to motion on a robot arm?
I think you have 3 options here.
1) A control the has the kinematics to control a robotic arm.
2) CAM software that can output the proper g-code to control the arm.
3) A "converter" program to convert conventional g-code into code to control the arm.
If you plan on coding this yourself, 3 may be the easiest option. Just guessing, though.