When everything goes to hell, the best plan of action is to reduce the problem to the lowest common denominator and then work your way back up.
I would suggest that you disconnect the gShield from the Arduino and hook up the USB cable to power the Arduino (leave the 24 volt power supply off as it only powers the stepper motors and fan). Run the Arduino IDE and bring up the serial monitor to see if your computer can connect to the Arduino.
Read through this thread and if you understand it then see if your computer will talk to grbl.
If not, post back and I’ll try to get you through it.