You can import G-code into Carbide Motion from a number of different packages pretty easily, the important thing is having the Post Processor set correctly because Carbide Motion is NOT just a plain vanilla GRBL implementation, so there may be some things that are different, since Carbide is doing some leg-work in the middle before sending your code over to the GRBL board in the Nomad/Shapeoko.
Rob’s recommendation (and what I’ve been successfully using) is a Mach3 post, which you can get fairly easily for most CAM packages. I’ve been using it out of Fusion360’s integrated CAM tools, which are from HSMworks. The only option I change on the Mach3 settings at this point is to turn “use radius” OFF to force it to send all arc movements as arcs and not use the radius notation.
As far as resources for understanding what various g-codes mean, I’ve posted previously on a few details, and would recommend this g-code reference from Tormach. The Nomad and Shapeoko don’t support all of the functionality offered there (drilling cycles, tapping, etc…) but pretty much all the stuff that is used is there.
I’d suggest checking out Fusion360 if you want some really powerful options since it’s free for hobbyists/educational use, and the 2D options in it are really good, once you get through the learning-curve!