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Is GRBL the very latest and best G Code interpreter out there for the DIY industry?
Please define "best".
Most RepRap firmwares depend on Arduino libraries, so, to port that to another platform you have to port these libraries, too. Teacup firmware is an exemption, it's independent from Arduino (uses is just as a convenient compile/upload tool). Even with Teacup you have to re-write the macros in pinio.h, arduino.h and similar.