Many G-code dialects contain subprogram calls, loops, and conditionals that seem like they'd permit a physical quine. The standard RepRap firmware breaks the rules by taking input:

https://reprap.org/wiki/G-code#M98:_Call_Macro.2FSubprogram

but Fanuc and other dialects allow M97 to call a subroutine chained to the end of a program.

G-code is like assembly language for CNC. It’s human readable, you can write it by hand if necessary, most machines will compile it internally to machine instructions, and there are a million different flavors with incompatible macros, etc. The flavor I’m most familiar with is Klipper’s interpreter, which will let you write macros, but that would technically be cheating since you could just stuff an entire program into a macro.