You might want to reach out to Al Baker or Kendall Clark at Stardog. Their product is more RDF centric than for generic graphs like Neo4j, but is in my opinion a much better product. They have a community version, and have open sourced several pieces, but it is commercial. https://www.stardog.com/docs/
If you want pure Open Source then Titan might be a good fit for Gitter. http://titan.thinkaurelius.com/