Try adding some publish task.

.gitlab-ci.yml

@@ -16,3 +16,13 @@ build:
     paths:
       - build
       - .gradle
+
+artifact:
+  stage: deploy
+  script: gradle publish
+  cache:
+    key: "$CI_COMMIT_REF_NAME"
+    policy: pull-push
+    paths:
+      - build
+      - .gradle

build.gradle.kts

@@ -53,6 +53,16 @@ publishing {
             from(components["javaPlatform"])
         }
     }
+    repositories {
+        maven {
+            url = uri("https://git.f3l.de/api/v4/groups/160/packages/maven")
+            name = "GitLab"
+            credentials(HttpHeaderCredentials::class.java) {
+                name = "Job-Token"
+                value = System.getenv("CI_JOB_TOKEN")
+            }
+        }
+    }
 }
 
 inline fun <reified T> getFromProperties(name: String): T = project.properties[name] as T

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.6.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.8.2-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists