you said you have rubygems proxy on nexus. let say localhost:8081/nexus/content/repositories/gems then use localhost:8081/nexus/content/repositories/gems/maven/releases as maven repository for your gems. then gradle will pick the gems from there.
yes you still need to use the "rubygems" as group name - always !
no need to do anything on nexus - just have a look at /content/repositories/rubygems/maven/releases and you will find a maven repository :)
undocumented feature :)
if you want to use this maven repo in group repo on nexus then you need to add a proxy maven repo pointing to this gem-artifact repo and add this maven proxy to your group - that is a bit annoying but the only way
yes the maven repo format is what gradle understands. there were some attempts to do this more under the hood but did not work out. for maven there is a transport extension now which can be configured with just "https://rubugems.org" and uses it directly but with gradle this can not be done within the plugin framework of gradle - unfortunately
Just a little feedback... if your docs were clearer, more people might use the project. Your 'quick start' includes a bunch of colouring text scripts that is about as far removed from real usage as ancient Egyptian history. Starting with "here is a Gemfile" and "here is build.gradle to do the same thing" is probably a better starting point.
For a brief bit, I was using a local maven repo for one of my gems, because it was a private gem and the server we host those on requires http basic auth. But, the latest jruby-gradle-plugin support basic auth in rubygems() repository urls.