Hi Chun,
It can be downloaded from "Release Download Files/Greenplum Advanced Analytics" in Pivnet (https://network.pivotal.io/products/pivotal-gpdb).
Hi Kushal,
Which version is supported by the sandbox:
PostGIS 2.1.5+pivotal.1 for RHEL 7
or
PostGIS 2.1.5+pivotal.1 for RHEL 6
Regards
You can check redhat release of the sandbox and download the specific RHEL Postgis version.