The GATEWAY_MODULES_ENABLED environment variable is only meaningful to a starting/running Ignition Gateway (i.e. it is not known to the installer). While typically leveraged in a Docker context, it is actually part of the Ignition platform and will also work in other OS's such as Windows. However, setting up environment variables for Windows services is not very straightforward.
This forum post shows the Windows Registry modifications for a given service definition and how to define environment variables.
This docs page shows what identifiers can be used in that comma-delimited list. As mentioned there, if you're on Ignition 8.1.x, there are separate "legacy" identifiers that must be used.