You can do a UNION ALL of the tables when creating a virtual dataset:
https://docs.dremio.com/working-with-datasets/virtual-datasets.html
If you’d like, you can create a reflection on this virtual dataset, and queries will then be rewritten to
use the reflection instead of pushing down into Greenplum.
If your query patterns frequently make use of subsets of the underlying partitions, you could experiment with creating a reflection on the physical datasets, and then a union as described above, and Dremio will decide which combination of reflections to use for a given query.