gcode writing for marlin

hi all,
i am trying to generate my custom gcodes with rhino-grasshopper. but i need to understand better gcode mechanism. i am using this page as a source: [reprap.org]

so there is this to start:

Gnnn :	Standard GCode command, such as move to a point

it says G0 is rapid and G1 is normal speed. but are we determining these speeds in firmware? how can i change speed to something between? or slower than G1? (or faster if G0 is not the max?)

more questions will come as i go further with writing gcodes,

thanks