There seems to be some kind of problem with giving custom arguments to GAnt target.
There where an el way, closure like way and now there is a mention about single parameter which must be a map or so.
Anyways, here are two alternative ways to solve the problem.

Use closures with parameters:

def sayMyName = { name ->
    println "Your name is $name "
}
 
target ( default: ''  ) {
    sayMyName ('Dimitri')
}

 
Remark. Closures are no targets, they are treated differently. There are interaction issues between closures and ant target. I see it as a risk factor.

Define global variables:

def name = ''
target ( sayMyName : 'Will say the name' ) {
  println "Your name is $name "
}
 
 
target ( default: ''  ) {
  name = 'Dimitri'
  sayMyName ()
}