The ASCII code for control x (aka ctrl-x, ^x, CAN) is hexadecimal 18, 0x18. I don’t know how or if gcode sender sends this. You should read the documentation on grbl, as once you send a ctrl x there is an unlock you will need to do to get the system out of alarm mode.
Your other alternative is just to wire the reset button in parallel to some place more convenient to get at it. On the later versions of grblshield there is a 2 pin connector on the board you can solder in. In earlier version you need to attache directly to the switch.