With the provided build.gradle file, the gradle build task fails with the following message:

FAILURE: Build failed with an exception.

What went wrong:

Unable to retrieve secret key from key ring file '.../opt4j/opt4j-benchmarks/void/.gnupg/secring.gpg' as it does not exist

Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

Commenting out the lines 74-77 in the build.gradle file in the opt4j folder solves the issue. I do not know whether this is a clean solution.