The release management in Gradle relies on version (and other) properties managed by gradle.properties file. You add all the relevant properties to the release management configuration, and the plugin will read and modify those properties in the gradle.properties file.
These are the basic steps that the plugin performs:
Change the gradle.properties with release values (before the build starts)
Trigger the Gradle build step (with optionally different tasks and options)
Commit/push changes to the tag (Subversion) or the release branch (Git)
Change the gradle.properties with next integration values