Unfortunately, at the moment Gosu is only available in Intellij (there is an eclipse plugin, but it's pretty badly out of date and a bit buggy)
You can find out more about installing that here: http://gosu-lang.org/intellij.html
Check src/main/GosuFeatures.gsp to check out some of the base Gosu features.
.gs is a gosu class
.gsp is a gosu program (the entry point, like main(String[] args))
.gsx is a gosu enhancement
When a project has the Gosu SDK selected and there is a typeloader jar on the classpath (in this case goson-0.3-SNAPSHOT.jar), it will automatically typeload any file that is appropriate for that loader. In this case, the project is picking up any .json files. Add your own .json files in to use them in a Gosu class/program