Probably, the below is already known to you. Never-the-less, I want to share this for those who do not.
I think we have all experienced it all: You have prepared a G-code file (Yes, you can cut and paste and alter) and want to see how it looks like. Also: How may times we have made a text that is different in font-shape/size and found out it did not fit? (I have destroyed quite a few CNC-mils already.)
An easy way is to use a G-code simulator. They are commercially available. But there is a cheap, easy solution which I use.
My CNC-router/Engraver is upstairs, but I do most of my prelim work in the living-room and watch the 9 o'clock news at the same time. How?
Just buy a 2 dollar Nano (See picture of Dummy-Nano), no need to solder the headers to it, just use it for simulation only. Xload your GRBL to it. Start GRBL-controller (= A little piece of software that is free to download. Free?, yes Free. That is as music to a Dutchmen's ear.)
Connect the Nano to comp., load file and begin. With GRBL-Controller, you see the starting-point (is it really in the very left-hand corner?), the max and min of Y and X in mm. You will see G-code commands that are not reckonised.
With best regards,