Gantry Sharpie Router Attached to Robot

Hi i need some help on how to code my project... I'm working on a robot that climbs walls and uses a sharpue to draw text on walls. The robot is able to freely move on the walls surface and the sharpie is attached to a gantry system that is mounted on the robot. The intention is that (using feedback from some distance sensors) the robot will position itself on the wall and then once in the correct position it'll stop moving. At this point, I would like the gantry system used (standard CNC like system using steppers and rails) to print one letter of the uploaded text. The robot then progresses to the next position, repeating the process until the text is complete.

I found the grbl firmware to be very convenient especially for my use, but I don't know how to separate the g code into each letter. Ideally, id have the user upload the text once (about 100 words lets say) and then I want the code to be able to distinguish first letter, second letter etc.

Anyone have any ideas on how to do this with G-code? can I have one big g code file but some how break it up into multiple files?

For works like that a graphical Pc tool could be used. There You select font, edit the text etc.
It's a bit apart from forum goals to create an editor outputting GRBL like You describe.
30 years ago SKF Gothenburg bought an engraving Pc based system. It allowed normal lines as well as text written on a circle.