Can someone explain to me what is the usefulness of using gtmpl files when developing for the juzu framework, instead of jsp files or others?
Gtmpl is a simple template engine inherited from Groovy Templates that works pretty well.
In addition Juzu also supports Mustache templating (http://juzuweb.org/reference/index.html#_the_mustache_template_engine) as logic-less template engine.
It is technically possible to integrate other template engines. However Juzu does not support (yet) JSP because they would not work with the Juzu Vert.x bridge.