Hi Bryan,
In answer to your questions, no, Pacer is not abandoned and is being actively used despite the low mailing list traffic.
I'm not sure what you mean by Ruby v4, unless you mean Rails 4. Pacer runs only on JRuby (a version of Ruby designed to run on the JVM) and should run fine on any recent version. If you are interested, you should be able to run your Rails app on top of the JRuby version of Ruby without any trouble, which would enable you to use Pacer together with Rails just fine.
Pacer is not an interface to Gremlin. Rather it is similar to Gremlin, but written in a more Ruby centric idiom. I'm not aware of any gems for using Gremlin from Ruby, though such a thing may well exist.
Hope that helps.
Cheers,
Darrick