getGeselectedID()
is a JavaScript function, so you don't have access of it in GSP tags (JavaScript is executed in the browser and GSP tags are parsed in the server side).
If you just want to submit a form with the id selected, you're already doing it, see the <input type="radio" />
.
The value will be available thought the name
attribute, in this case agendaGroup
.
def prikDatum(Aanvraag aanvraagInstance){
println params.agendaGroup
}