Install an extension.
gel extension install extension [options]
The name of the extension to install.
See Connection options for global options.
Install the postgis extension:
postgis
$
gel extension install postgis
Found extension package: postgis version 3.4.3+6b82d77 00:00:03 [====================] 22.49 MiB/22.49 MiB Extension 'postgis' installed successfully.
