In Gremlin 2.3.0, there are three new steps:
linkIn
linkOut
linkBoth
Please see GremlinDocs (http://gremlindocs.com) for more information on how to use them.
http://gremlindocs.com/#transform/linkboth-in-out
Note that these pipes yield a sideEffect (the edge generated) so you can cap the pipe if you need to get the edge.