Hint: This is the main page for G-code

Key Concept

Move in X Y Z directions. Ex: G0 X100 Y100 Z100
To use Marlin - observe what happens in Start Gcode in Cura - and then add your commands in a Gcode file.
Do an exercise: get out a text editor, and move your machine around. Move up so you don't hit anything. On Universal, stay within 100 mm just to get the hang of it.

Widely Used in Marlin Firmware

G1 E10 - spit 10 mm of filament
M92 - sets feed rate, such as extruder - M92 E408. Set this in Start G-Code to avoid any software ambiguity
G92 - resets coordinates to given values [1]
G90 - set absolute positioning
G91 - set relative positioning
M106 - Turn fan on. 0-255 for fan speed. No argument means turn it on. M107 to turn fan off.
Homing - G28 ; home all axes (X, Y, and Z); G28 X Y ; home X and Y axes; G28 Z ; home Z axis only
M18 - disable steppers
G code cheatsheet - [2]

M119 - status of endstops (triggered, not triggered)
M503 - Print the current settings – Not the settings stored in EEPROM.
M501 Read all parameters from EEPROM. (Or, undo changes.). Note that EEPROM must be enabled for EEPROM values to be used. See EEPROM Settings

When moving printer in the Cura command line - use G92 without parameters to set all axes to zero
G1 will always work when coordinates are reset to zero, otherwise if you are alraedy at say E25, G1 E25 will not move the extruder.