The Gremlin Tutorial is a good starting point to learn about Gremlin.
The Tutorial examples use the Gremlin console. You'll find corresponding java examples in simplegraph-core TestGremlinTutorial
You might want to try these out in your favorite IDE e.g. SimpleGraph-Installation#Eclipse
The next step is to go thru the example in Practical Gremlin: An Apache TinkerPop Tutorial by Kelvin Lawrence see also https://github.com/krlawrence/graph