G20 means "all moves starting now have their units specified in inches ( imperial units )".
G20's evil twin is G21, which means "all moves starting now have their units specified in millimeters ( metric units )", which is the default when Smoothie starts.
An inch is 25.4 millimeters.
The command is used as such :
Which means : units are now imperial until another G-code changes it ( modal command )
These resources are used as references for Gcode :