I am wondering How can I pass Environment setting to a http4k application using gadlew?

If you want to use typesafe 12-factor configuration then have a look at the examples repo. As for passing values from Gradle - there's nothing special about http4k in that regard, so the Gradle docs are your best bet. 🙃

btw How can I switch to Environment.defaults() if no env has been provided? I mean something similar to Spring Boot properties app.foo=${BAR_ENV:foo}

You need to provide an Environment. There are functions on the companion object to give you these