GPGSetup

When publishing a library to a maven repository (like Maven Central), you need to sign the built jar file(s).
The signing process is done with a GPG key which needs to be published to a key server (hkps://keys.openpgp.org in my case).

When using Windows, you can download the GPG for Windows from here.
When using Linux, you'll figure it out yourself :).

After installing GPG and creating a key, you can export the secret key to a file:

gpg --export-secret-key > secretkey.gpg

The secret key can then be used to sign the jar file(s) when publishing:

signing.keyId=A1B2C3D4
signing.password=123456
signing.secretKeyRingFile=secretkey.gpg

The signing.keyId is the last 8 characters of the key id.

You can find more information in the official Gradle documentation here.