Not sure what Inventables has done w/ their fork of Grbl — doubt that it’s much beyond setting defaults appropriately for their machine and software — nothing more should be necessary. They fund development and should be able to get any internals which they want.
Not understanding what you mean by
There isn’t really any software which directly allows that — I can’t think of anything for instance which directly supports the plane-switching commands for arcs, nor anything which uses G-code’s facility to move on a diagonal along the Z-plane — the communication / control app which exposes the most of G-code’s capabilities is bCNC. If you’re running Windows, you might find GrblGru of interest (linked from the front page of the Shapeoko wiki and available from: Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable )
One or the other (or both) of them has integrated a G-code processor which makes directly writing G-code much less tedious than Grbl otherwise allows — Grbl is intentionally kept simple with the idea of only supporting the G-code which a CAM program will output – no support for loops, pointers, &c. Directing writing G-code and limiting oneself to what Grbl affords is an exercise in tedium: Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable