organizationFolder("Org1/Org2") {
    organizations {
        gitLabSCMNavigator {
            projectOwner("Org1/Org2")
            credentialsId("cee-gitlab-token")
            serverName("cee-gitlab-server")
            traits {
                gitLabBranchDiscovery {
                    strategyId(3) // discover all branches
                }
                originMergeRequestDiscoveryTrait {
                    strategyId(1) // discover MRs and merge them with target branch
                }
                logComment {
                    logSuccess(true)
                }
                mrTriggerComment {
                    commentBody("jenkins rebuild")
                    onlyTrustedMembersCanTrigger(true)
                }
            }
        }
    }
    // "Traits" ("Behaviours" in the GUI) that are NOT "declarative-compatible"
    // For some 'traits', we need to configure this stuff by hand until JobDSL handles it
    // https://issues.jenkins.io/browse/JENKINS-45504
    configure {
        def traits = it / navigators / 'io.jenkins.plugins.gitlabbranchsource.GitLabSCMNavigator' / traits
        traits << 'io.jenkins.plugins.gitlabbranchsource.ForkMergeRequestDiscoveryTrait' {
            strategyId(2)
            trust(class: 'io.jenkins.plugins.gitlabbranchsource.ForkMergeRequestDiscoveryTrait$TrustPermission')
        }
    }

    projectFactories {
        workflowMultiBranchProjectFactory {
            scriptPath 'Jenkinsfile'
        }
    }

    // "Orphaned Item Strategy"
    orphanedItemStrategy {
        discardOldItems {
            daysToKeep(10)
            numToKeep(5)
        }
    }

    // "Scan Organization Folder Triggers" : 1 day
    // We need to configure this stuff by hand because JobDSL only allow 'periodic(int min)' for now
    triggers {
        periodicFolderTrigger {
            interval('1d')
        }
    }
}

Checking merge requests..

Checking merge request !9
ERROR: [Wed Aug 03 07:08:00 UTC 2022] Could not fetch branches from source io.jenkins.plugins.gitlabbranchsource.GitLabSCMNavigator::https://gitlab.example.com::Org1/Org2::Org1/Org2/my-repo
[Wed Aug 03 07:08:00 UTC 2022] Finished branch indexing. Indexing took 0.99 sec
FATAL: Failed to recompute children of Org1 » Org2 » Org1/Org2/my-repo
org.gitlab4j.api.GitLabApiException: 404 Project Not Found
    at org.gitlab4j.api.AbstractApi.validate(AbstractApi.java:633)
    at org.gitlab4j.api.AbstractApi.get(AbstractApi.java:213)
    at org.gitlab4j.api.ProjectApi.getProject(ProjectApi.java:673)
    at io.jenkins.plugins.gitlabbranchsource.GitLabSCMSource.retrieve(GitLabSCMSource.java:417)
Caused: java.io.IOException: Failed to fetch latest heads
    at io.jenkins.plugins.gitlabbranchsource.GitLabSCMSource.retrieve(GitLabSCMSource.java:538)
    at jenkins.scm.api.SCMSource._retrieve(SCMSource.java:373)
    at jenkins.scm.api.SCMSource.fetch(SCMSource.java:283)
    at jenkins.branch.MultiBranchProject.computeChildren(MultiBranchProject.java:641)
    at com.cloudbees.hudson.plugins.folder.computed.ComputedFolder.updateChildren(ComputedFolder.java:278)
    at com.cloudbees.hudson.plugins.folder.computed.FolderComputation.run(FolderComputation.java:166)
    at jenkins.branch.MultiBranchProject$BranchIndexing.run(MultiBranchProject.java:1032)
    at hudson.model.ResourceController.execute(ResourceController.java:99)
    at hudson.model.Executor.run(Executor.java:431)
Finished: FAILURE