Here's the money quote:
Spark focuses on being as simple and straight-forward as possible, without the need for cumbersome (XML) configuration, to enable very fast web application development in pure Java with minimal effort.
SparkGS is an extension of SparkJava that leverages the many advantages of the Gosu programming language over Java.
It allows you to define your spark program in a way that is closer to the SinatraRB approach (in a simple program file) while giving you a few higher-level route definition tools for things like REST-ful routes. And since Gosu offers type-safe templating out of the box, there is no need to install an awkward 3rd party templating system.
SparkGS makes it dead simple to get a JVM based web-server going and it is trivially easy to deploy to Heroku, making SparkGS the easiest way we know of to get a JVM application up and going in the cloud.