I am confused about where Gradle fits into the build lifecycle. Is it trying to be a Jenkins or GoCD CI/CD?
no - gradle is a free and open source build tool (like ant, maven,…). It can be used in any CI environment, like Jenkins or TeamCity,… as they provide plugins to start gradle builds, but does not trigger builds.
In addition to that Gradleware also provides services around gradle itself to master and to analyse company builds, which have to be paid.
Hope that helps,