The following is a list of the officially supported g-codes in GRBL. Helpful for debugging if you are trying to load a generic g-code file and are having issues.
Grbl is for three axis machines. No rotation axes.
Supported G-Codes in v0.9i G38.3, G38.4, G38.5: Probing G40: Cutter Radius Compensation Modes G61: Path Control Modes G91.1: Arc IJK Distance Modes Supported G-Codes in v0.9h G38.2: Probing G43.1, G49: Dynamic Tool Length Offsets Supported G-Codes in v0.8 (and v0.9) G0, G1: Linear Motions G2, G3: Arc and Helical Motions G4: Dwell G10 L2, G10 L20: Set Work Coordinate Offsets G17, G18, G19: Plane Selection G20, G21: Units G28, G30: Go to Pre-Defined Position G28.1, G30.1: Set Pre-Defined Position G53: Move in Absolute Coordinates G54, G55, G56, G57, G58, G59: Work Coordinate Systems G80: Motion Mode Cancel G90, G91: Distance Modes G92: Coordinate Offset G92.1: Clear Coordinate System Offsets G93, G94: Feedrate Modes M0, M2, M30: Program Pause and End M3, M4, M5: Spindle Control M8, M9: Coolant Control