Fields should allow array types. In environment variables these can be CSV or use a custom delimiter. The "interior" validator should be definable.

Maybe something like

{
  defaultFeatureFlags: g.array(g.string()).delimiter(/\s/),
  listenPorts: g.array(g.number()).default([8080]),
}

And the matching environment variables:

DEFAULT_FEATURE_FLAGS="new-feature old-feature demo-feature"
LISTEN_PORTS="8080, 8081"

Notice the allowable whitespace in LISTEN_PORTS.