manual_gate.groovy

def LABEL_ID = "yourappname-${UUID.randomUUID().toString()}"
def BRANCH_NAME = "<Your branch name>"
def GIT_URL = "<Your git url>"
// Start Agent
node(LABEL_ID) {
    stage('Checkout') {
        doCheckout(BRANCH_NAME, GIT_URL)
    }
    stage('Build') {
        ...
    }
    stage('Tests') {
        ...
    }    
}
// Kill Agent
// Input Step
timeout(time: 15, unit: "MINUTES") {
    input message: 'Do you want to approve the deploy in production?', ok: 'Yes'
}
// Start Agent Again
node(LABEL_ID) {
    doCheckout(BRANCH_NAME, GIT_URL) 
    stage('Deploy') {
        ...
    }
}
def doCheckout(branchName, gitUrl){
    checkout([$class: 'GitSCM',
        branches: [[name: branchName]],
        doGenerateSubmoduleConfigurations: false,
        extensions:[[$class: 'CloneOption', noTags: true, reference: '', shallow: true]],
        userRemoteConfigs: [[credentialsId: '<Your credentials id>', url: gitUrl]]])
}