The default target, i.e. the target executed when no target is specified on the command line is specified by defining the target 'default'. So for example:
target ( 'default' , 'The default target.' ) { aTarget ( ) }
This assumes aTarget exists!
Rather than requiring that the above mechanism is used, Gant provides a shortcut:
setDefaultTarget ( aTarget )
What this actually does is:
target ( 'default' : 'aTarget' ) { aTarget ( ) }
The convention of using the target name as the description is for creating the string at the end of the project help output (-p, --projecthelp, -T, --targets options to Gant).
The parameter to the setDefaultTarget call can be either the name of a target or a string that evaluates to the name of a target. So for the above example we could have:
setDefaultTarget ( 'aTarget' )
This allows great flexibility, possibly needless 
, in coding up defaults. For example:
defaultTargetName = 'aTarget' setDefaultTarget ( defaultTargetName )