The G-Code commands supported by Marlin 3D printing firmware are:

Implemented Codes
G0 Rapid Motion
G1 Coordinated Movement X Y Z E
G4 Dwell S or P
G10 Retract filament according to settings of M207
G11 Retract recover filament according to settings of M208
G28 Home all Axis
G90 Use Absolute Coordinates
G91 Use Relative Coordinates
G92 Set current position to coordinates given

RepRap M Codes
M0 Unconditional stop. Wait for user to press a button on the LCD (Only if ULTRA_LCD is enabled)
M1 Same as M0
M104 Set extruder target temp
M105 Read current temp
M106 Fan on
M107 Fan off
M109 Wait for extruder current temp to reach target temp.
M114 Display current position

Custom M Codes
M17 Enable/Power all stepper motors
M18 Disable all stepper motors; same as M84
M20 List SD card
M21 Init SD card
M22 Release SD card
M23 Select SD file (M23 filename.g)
M24 Start/resume SD print
M25 Pause SD print
M26 Set SD position in bytes (M26 S12345)
M27 Report SD print status
M28 Start SD write (M28 filename.g)
M29 Stop SD write
M30 Delete file from SD (M30 filename.g)
M31 Output time since last M109 or SD card start to serial
M42 Change pin status via gcode
M80 Turn on Power Supply
M81 Turn off Power Supply
M82 Set E codes absolute (default)
M83 Set E codes relative while in Absolute Coordinates (G90) mode
M84 Disable steppers until next move, or use S to specify an inactivity timeout, after which the steppers will be disabled. S0 to disable the timeout.
M85 Set inactivity shutdown timer with parameter S. To disable set zero (default)
M92 Set axis_steps_per_unit same syntax as G92
M114 Output current position to serial port
M115 Capabilities string
M117 Display message
M119 Output endstop status to serial port
M140 Set bed target temp
M190 Wait for bed current temp to reach target temp.
M200 Set filament diameter
M201 Set max acceleration in units/s^2 for print moves (M201 X1000 Y1000)
M202 Set max acceleration in units/s^2 for travel moves (M202 X1000 Y1000) Unused in Marlin!!
M203 Set maximum feedrate that your machine can sustain (M203 X200 Y200 Z300 E10000) in mm/sec
M204 Set default acceleration: S normal moves T filament only moves (M204 S3000 T7000) in mm/sec^2 also sets minimum segment time in ms (B20000) to prevent buffer underruns and M20 minimum feedrate
M205 advanced settings: minimum travel speed S=while printing T=travel only, B=minimum segment time X= maximum xy jerk, Z=maximum Z jerk, E=maximum E jerk
M206 Set additional homeing offset
M207 Set retract length S[positive mm] F[feedrate mm/sec] Z[additional zlift/hop]
M208 Set recover=unretract length S[positive mm surplus to the M207 S*] F[feedrate mm/sec]
M209 S<1=true/0=false> enable automatic retract detect if the slicer did not support G10/11: every normal extrude-only move will be classified as retract depending on the direction.
M220 S– set speed factor override percentage
M221 S– set extrude factor override percentage
M240 Trigger a camera to take a photograph
M301 Set PID parameters P I and D
M302 Allow cold extrudes
M303 PID relay autotune S sets the target temperature. (default target temperature = 150C)
M400 Finish all moves
M500 Stores paramters in EEPROM
M501 Reads parameters from EEPROM (if you need reset them after you changed them temporarily).
M502 Reverts to the default “factory settings”. You still need to store them in EEPROM afterwards if you want to.
M503 Print the current settings (from memory not from eeprom)
M999 Restart after being stopped by error