The content of a gendoc tag corresponds to a script written in Acceleo language.
Acceleo syntax to display names of all packages
Example: display names of all packages
[for (p:Package | Package.allInstances())]
[p.name/]
[/for]
Gendoc syntax to display names of all packages
<context model='D:/…/myModel.uml'/>
<gendoc>
[for (p:Package | Package.allInstances())]
[p.name/]
[/for]
</gendoc>