To upload a release to Maven Central you first must follow the OSSRH guide to register an account and receive permissions to the org.mini2Dx project group.
Once you've registered, set up your PGP key per the guide here and add the required details to ~/.gradle/gradle.properties.
Then you can build and publish a release with the following command:
./gradlew -Prelease clean build uploadArchives closeAndReleaseRepository