First of all, welcome to the Thor community 😉
I still have to generate the control documentation, which would include the GRBL mods and how to use the custom GCode. It is on my to-do-list haha.
Answering your question quickly, the way I control Thor via GCode is using forward kinematics. It means that I send to the control board the values of the angles for the 7 motors.
G01 A10 B20 C20 D30 X40 Y50 Z60
This Gcode will move the 1st articulation (A) 10 degrees clockwise, the second (B & C as there are 2 steppers in this articulation) 20º CW, the third (D) 30º CW, 4th (X) 40º CW , 5th (Y) 50º CW and 6th (Z) 60º CW
The Inverse kinematic calculations are already done but it’s hard to implement into an Arduino, due to the calculations per second that the microprocessor has to do.
Hope it helps 😉