Gosu, like Scala, Kotlin and Ceylon, was built to be statically typed from the ground up and thoroughly tested and documented by many developers, with dynamic typing added to some of them as an afterthought, like the Dynamic type in Scala.

Groovy, like Clojure, Beanshell, Jython, JRuby, Rhino and Nashorn, was built to be dynamically typed from the ground up, with static typing added opportunistically in version 2.0 by a single developer and requiring klunky annotation syntax.

Gosu actually went public before Kotlin and I think Gosu helped inspire Kotlin in a couple of ways.

While I'm glad Gosu keeps chugging along, I can't see myself choosing it over Kotlin.