It's Apache 2.0, so free as in beer and free as in speech.

https://github.com/greenplum-db/gpdb/blob/master/LICENSE

It's released under the Apache2 license. So, IANAL but yes. They do have some kind of a commercial/support offering on top of it.

Apache license, so yes, free to use and modify for whatever purpose.

It says it's Apache 2 licensed, so it should be free-as-in-freedom, even for enterprise use.