To get Briar GTK into Debian (or to at least provide a .deb installation method), we need to create a Debian package for Briar Headless that builds everything from source. briar!1252 (closed) brought at least a binary package.
With briar!1252 (closed) I was able to create a simple binary package for Debian. The package takes the briar-headless.jar built locally (with the usual Internet-connected Gradle build style) and then packages that .jar file.
This method doesn't comply with Debian's Main policy. However, it currently doesn't seem possible to package briar-headless from source for Debian, as it still lacks Kotlin.
Nicochanged title from Create Debian Package for Briar Headless to Create Debian Source Package for Briar Headless
changed title from Create Debian Package for Briar Headless to Create Debian Source Package for Briar Headless
briar-headless in Debian main is not only blocked by Kotlin, but also by Gradle
Additionally, each dependency of briar-headless needs to be packaged separately
Plans to get into next Debian stable release were given up
Instead, we'll wait until Debian offers recent versions of Kotlin and Gradle and a sponsor/volunteer for packaging briar-headless in Debian main shows up
Efforts will now be focused on providing an official APT repository of The Briar Project (#3)