Greenplum Database is based on PostgreSQL (was forked from PG 8.2), and features a massive parallel processing system (MPP) to run SQL queries on very large data sets. The code base is licensed under the Apache 2.0 license, and available on GitHub. You can fork the project from there, or submit patches and new features.
One of the main goals of the engineering team is to merge the existing code base with a recent PostgreSQL version. Although many features from newer PostgreSQL versions made it into Greenplum, there are many differences in terms of code. Also Greenplum offers unique features (new query optimizer, SQL support for partitioning, append-optimized tables, columnar storage, storage compression and many more), which over time will be ported to PostgreSQL and submitted for community review.