I'm fairly new to the language, as it hasn't been mentioned much since it's public release several years ago. The language takes many queues from Ruby; for example, "enhancements" are similar to monkey patching in Ruby. The syntax, semantics, and standard library (especially in collections) are also familiar.

e.g.:

Ruby

[1, 2, 3].map { |x| 2 * x }

Gosu

{1, 2, 3}.map (\x -> 2 * x)

Unlike Ruby, Gosu provides a static type system which includes type inference and structural typing (similar to Go's interfaces). It also includes an open type system (http://devblog.guidewire.com/2010/11/18/gosus-secret-sauce-t...), which allows first-class representations of user created types at compile time – no code generation is needed. F# programmers would find many similarities between Gosu's open type system and F#'s type providers.

Like most JVM languages, you can leverage Java compatibility to use all of the third-party libraries you want. Compared to Scala, Gosu tends to eschew complexity for simplicity and pragmatism. Their implementation of generics is a good example of this.

And of course, Gosu just has a lot of features that make it enjoyable to work with. Such as the null-safe invocation operator (?.), elvis operator (?:), type variable reification, terse syntax, etc.