@whitequark pretty sure Spring has been generating executable JARs for a while (the docs seem to agree https://docs.spring.io/spring-boot/specification/executable-jar/index.html) so running `./gradlew assemble` should give you one somewhere inside `<module>/build/outputs`.
I haven't worked with Spring in a bit so I'm fuzzy on the details, let me know if you can't find it there and I'll set up one of their templates to check.