According to Maven Central requirements, these are needed:

• JavaDoc
• File Checksums (.asc) 
• Metadata
  • Correct Coordinates
  • Project Name
  • Description
  • URL
• License Information
  • https://central.sonatype.org/publish/requirements/#license-information 
• Developer Information
  • https://central.sonatype.org/publish/requirements/#developer-information 
• Source control system information
  • https://central.sonatype.org/publish/requirements/#scm-information
• Example .pom file:
  • https://central.sonatype.org/publish/requirements/#a-complete-example-pom
• Signing 
  • https://docs.gradle.org/current/userguide/publishing_signing.html#publishing_maven:signing

Add in the publication {}: or add them in afterEvaluate { publication {}}