Gremlin is agraphdb query language supported by AWS Neptune and Neo4j. Gremlin offers lower-level graph traversal execution control than OpenCypher at the cost of reduced readability and increased complexity.
From StackOverflow:
Gremlin is a Groovy DSL. Using it through its Neo4j REST endpoint allows full, unfettered access to the underlying Neo4j Java API.
Gremlin is built over Groovy, so you can actually use it as a generic way to get Neo4j to execute ‘Java’ code and perform various tasks from the server, without having to take the HTTP hit from the REST interface. Among others, Gremlin will let you modify data.
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However, when all I want is to query data, I go with Cypher as it iebs more readable and easier to maintain. Gremlin is the fallback when a limitation is reached.